As the Terrapins men’s lacrosse team convened in the locker room before its game Saturday, the squad realized an unexpected visitor had tagged along.

Fionn Crimmins, a 5-year-old boy with acute lymphoblastic leukemia whom the Terps drafted through Team Impact this season, had warmed up with the team. He dashed around on the Maryland Stadium turf, participating in pregame stretches and making the rounds for high-fives.

But “unbeknownst to anybody,” Fionn followed his teammates off the field and “was just running around the locker room,” coach John Tillman said with a smile after the game.

It was one of Fionn’s highlights during an afternoon the Terps dubbed “Team Impact Day.” The team honored Fionn and his family and held an auction to raise money for the program, which pairs kids with life-threatening and chronic illnesses with local college teams.

Since the fall, the Terps have befriended and supported Fionn while he completes the maintenance phase of his cancer treatment. The Terps said the young boy with shaggy blond hair and an ear-to-ear smile has inspired them, too.

“There’s definitely a reverse effect with him,” midfielder Bryan Cole said. “We’re trying to pick him up and keep his spirits up, but at the same time, when he comes in the locker room, when he’s on the field … it just makes us tremendously happy and definitely gives us a different perspective when we’re playing the game.”

Once Fionn’s parents took him out of the locker room so the Terps could focus on High Point, he watched his teammates jump out to a 2-0 lead before entering halftime in a two-goal hole.

That didn’t erase Fionn’s smile, though, as he stood with his family at midfield waving to cameras and the crowd as the announcer promoted Team Impact and the Terps’ involvement.

During the break, Fionn — clad in a red shirt with a big block “M” and black sleeves — also returned to the locker room before standing in the tunnel to slap more high-fives as his teammates emerged for the second half.

“When he came into the locker room — right before we were going out in the tunnel — I got chills just seeing him,” attackman Dylan Maltz said. “It makes you want to play that much harder knowing we have someone like that on the sidelines.”

Fionn and his dad, Conor Crimmins, retreated to their seats for the second half to eat hot dogs and chocolate chip cookies while watching the Terps come back for a 15-10 win.

But as the clock wound down on the Terps’ season opener, Fionn had one thing left to do. He wanted to make sure he was back on the field in time to shake hands with the Panthers players.

“Aside from playing in the game,” Conor Crimmins said, “he did every single thing.”

Saturday wasn’t Fionn’s first time in Maryland Stadium. During the preseason, the Crimmins family attended a team dinner in Tyser Tower, where Fionn ate with the Terps, played tag throughout the media suite and rode the elevator for fun.

He also came to a preseason practice, shooting on goal and passing with the players. It was extra practice for Fionn, who started playing lacrosse this winter; he hadn’t been introduced to the sport before joining the Terps.

His parents said he’s progressed this season from confusing the words “hockey” and “lacrosse” to looking forward to starting a spring league after Easter.

Perhaps that’s why Fionn was so comfortable after Saturday’s game, racing around the field to collect extra balls with a stick almost twice his size.

Then he found attackman Louis Dubick, one of the Terps who has formed an especially close bond with Fionn. Dubick was one of three players who went bowling on campus with Fionn in November.

And before returning to College Park in January for the spring semester, Dubick drove from his home in Potomac to the Crimmins’ house in Kensington with two pints of ice cream — chocolate and vanilla, Fionn’s favorites.

Dubick and Fionn, along with Conor Crimmins and Fionn’s 2-year-old brother, Deaglan, had a boys’ night, complete with the dessert, Legos and magnet tiles.

Fresh off the first game of his college career, Dubick posed for a picture with Fionn. The image shows Dubick hoisting Fionn up on his shoulders, but the Terps say the young boy’s presence does the heavy lifting.

“Given all he’s been through at his age, it seems like a lot of the worries we have seem a bit trivial, [so] when you see what he’s gone through and what he’s battling, you know, coming back from being down a couple goals really isn’t that big a deal,” Tillman said. “I told him he was the magic.”