Jayson Tatum saw himself in a young cancer patient. Their bond took everyone by surprise.

On Sunday, a basketball that started the day in the TD Garden equipment room became a keepsake after Jayson Tatum erupted for 51 points using it in the Celtics’ Game 7 win over the Philadelphia 76ers in their Eastern Conference semifinal. Celtics owner Wyc Grousbeck presented the ball to Tatum afterward, but the star knew it would be more meaningful to someone else.

Hanging out with Tatum during the post-game celebrations was 10-year-old Xavier Goncalves, who has undergone three surgeries to remove a cancerous tumor from his eye, 24 rounds of chemotherapy, and months of radiation treatments.

Over the last two months, Tatum and Goncalves have become friends. Tatum has invited him to games, exchanged text messages with him, and visited him at his home. So pulling Xavier into the locker-room revelry and giving him the ball after the most impressive performance of Tatum’s life was an easy choice.

“I could tell he didn’t really know what to do,” Tatum said. “I was like, ‘Man, you’re in the Garden with me. You can go anywhere you want. Come on in.’ ”

Xavier shows off the ball Jayson Tatum gave him after Tatums scored 51 points — an NBA Game 7 record — on Sunday. Tatum’s sweatbands sit in front of the ball, and another signed by Kevin Garnett.JESSICa RINaLDI/GLOBE STaFF

a difficult diagnosis

Late last summer, Samantha Bowditch noticed that her son’s Xavier’s left eyelid was drooping. Multiple doctors mostly said the problem would resolve itself, but it didn’t. When Samantha could feel a small lump in the corner of the eye in early November, she brought Xavier to Boston Children’s Hospital, where they diagnosed him with rhabdomyosarcoma, a soft-tissue cancer common in children.

The tumor under Xavier’s eyelid had actually grown behind his eye and into his nasal cavity. He underwent an initial surgery to remove the tumor and spent several months making weekly 30-mile commutes from his family’s Raynham home to the Dana-Farber Cancer Institute for chemotherapy.

Then in February, he started daily radiation treatments at Massachusetts General Hospital and he and his mother moved into an apartment at Christopher’s Haven, a home and community for young cancer patients and their families. They have been there since.

Samantha Bowditch and Xavier Goncalves have lived at Christopher’s Haven since February. Sam said Jayson Tatum has come to see the pair at the home, including a visit that lasted three hours.JESSICa RINaLDI/GLOBE STaFF

During activities at the center, Xavier often talked about how he loved Tatum and the Celtics but had never been to a game. a staff member reached out to the Celtics, who invited Xavier, his mother, and older brother, Carter, to the april 7 game against the Toronto Raptors.

Celtics vice president of marketing Nicole Federico brought the family courtside for pregame warm-ups, and Tatum walked over and handed Xavier a pair of his new signature Nike shoes.

“You’ve got to wear these tonight to give me some good luck,” Tatum said.

Xavier wore the shoes, the Celtics won, and Xavier took them off once the game ended to keep them safe. It was a wonderful night and a nice gesture. Xavier figured it would end there.

“Some celebrities might meet someone and then forget about them,” he said. “But I’m glad we were able to make a bond.”

‘Do you get nervous?’

Tatum was impressed by Xavier’s strength and courage, and drawn to his shy personality, which reminded him of himself.

“He was cool, he was quiet,” Tatum said. “I just felt a connection.”

The family’s story struck a chord with Tatum, whose 5-year-old son, Deuce, is a constant presence at Celtics games. Tatum thought about what it would be like to go through such a challenging time with his own family, and it crushed him.

“as parents, you do everything you can to provide and protect and make sure your child is safe, and things like this are uncontrollable,” Tatum said. “It must be just so tough on his mom.”

Sam holds a photo from one of Jayson Tatum’s visits to Christopher’s Haven. Tatum originally met Xavier very briefly while at TD Garden, and then decided to pay him a visit so they could talk. Tatum said Goncalves reminds him of himself.JESSICa RINaLDI/GLOBE STaFF

So a few days after the brief TD Garden meeting, Tatum texted Federico and said he wanted to visit Xavier in a less-hectic setting. He went to Christopher’s Haven a little more than a week later.

Tatum and Xavier hung out in the family’s apartment while Xavier peppered Tatum with questions.

Who are your favorite NBA players ever? (Michael Jordan, Kobe Bryant, LeBron James, Magic Johnson)

How do you like to spend your time with Deuce? (Swimming is the best)

What do you miss most about living a normal life? (Grocery shopping)

Tatum asked Xavier about his own challenges, about missing his friends, about waking up and walking into a hospital to receive radiation treatments.

“I asked him, like, ‘Do you get nervous?’ ” Tatum said. “and he just said, ‘Nah, because I know it’s going to help me. I know I have to do this.’ and his mom and everybody around him encourage him. I thought that was cool, because I’m like, ‘[Expletive], I get nervous before games going out in front of these people, and for you to be 10 years old and show that courage is impactful.’ ”

They went to the building’s activity area, where Tatum grabbed rebounds for Xavier on a small basketball hoop and added his palm print — in green paint, of course — to a collection of those of patients and families on a large mural. The visit lasted nearly three hours.

Jayson Tatum put his handprint on the wall — using green paint, of course — at Christopher’s Haven, a home for young cancer patients and their families run by Mass General.GONCaLVES FaMILY

‘That was the best day of my life’

Xavier and his mother have texted Federico updates on his progress over the last month, including a video of him ringing a bell to commemorate his final radiation treatment, and Federico sent them to Tatum, who shared support and well-wishes in return.

When the Celtics’ series against the 76ers was extended to a seventh game last week, Tatum wanted Xavier there.

Xavier was back near the court for warm-ups, and this time Tatum was no stranger. He hugged Bowditch and wished her a happy Mother’s Day. He asked Xavier why he wasn’t wearing the signed shoes he gave him last time, and Xavier explained he just couldn’t risk getting them dirty.

“Yo, we’re gonna win tonight,” Tatum told Xavier. “after the game, do you want to come to the locker room?”

He didn’t need to ask twice. Tatum took the court and poured in an NBA Game 7-record 51 points, leading the Celtics to a 112-88 win that sent them to the conference finals.

afterward, Xavier and his mother were ushered to a tunnel near the court, where people were lined up for various greetings, but nothing nearly as private as a locker-room visit. When Tatum arrived, he told Xavier to come with him, and off they went.

Xavier walked around and high-fived the Celtics before sitting near the locker next to Tatum’s. They talked about the game, and whether Xavier could keep his bare feet in ice water like Tatum was doing.

Bowditch was in the hallway outside, and as time passed she called in a few times to make sure her son wasn’t overstaying his welcome. Tatum insisted that he was not.

“He was just one of the boys,” Tatum said.

Xavier Goncalves was just “one of the boys” hanging out in the locker room after the Celtics advanced to the Eastern Conference Finals.GONCaLVES FaMILY

Xavier played it cool in front of the Celtics, but he was bursting inside. The truth came out during the ride home.

“He was absolutely in awe,” Bowditch said. “He was like, ‘Mom, that was the best day of my life.’ ”

Tatum wants Xavier to return for another game during these playoffs, perhaps the NBA Finals, pointing out that the Celtics are 2-0 with him in attendance.

Xavier recently underwent another round of scans and is expected to receive the results soon. If the news is good, the family is hopeful that Xavier will be able to return home and just need quarterly check-ups. But Xavier insists that if he needs to fight again, he will.

“Even if it’s a bad MRI, I was able to go through the treatments before that,” he said. “I’d just have to pretty much do what I was doing before, just try to get through my treatments.”

Except this time, he would have one of the best basketball players in the world on his side.

Xavier Goncalves ties the sneakers he received from his new friend, Jayson Tatum.JESSICa RINaLDI/GLOBE STaFF