CHESTNUT HILL — As Thomas Castellanos warmed up in the northeast corner of the Alumni Stadium game field Saturday night, an unassuming figure approached him.

In any other setting, this man — early 60s, gray hoodie, baseball cap, blue jeans, running shoes — would have passed for your standard, everyday dad. But at Boston College, he’s royalty.

It was Doug Flutie, stopping by to share some words of wisdom with another diminutive, dual-threat BC quarterback.

The two spoke only briefly, but Castellanos took the Heisman Trophy winner’s message to heart.

Four hours later, he engineered a late-game comeback that would have made Flutie proud, hitting Lewis Bond for a 42-yard game-winning touchdown to knock off Michigan State 23-19 in the 11th annual Red Bandanna Game.

“It was amazing,” Castellanos said. “During pregame warmup, (Flutie) came up to me and was like, ‘You got this. Lead your guys.’ Ex-Heisman winner, a legend here. You walk around, and you’ve still got his jersey sold in the bookstores and you still see people wearing them.

“It’s just kind of motivation, for a guy like that to come back and give words of encouragement or just details or things here and there to help my game or help me as a leader on and off the field. It’s amazing.”

The touchdown to Bond wasn’t as dramatic as Flutie’s famed Hail Mary in Miami (there was still 1:28 left on the clock), nor was the stage nearly as prominent (the Eagles and Spartans both were unranked). And unlike Flutie in that iconic 1984 victory, Castellanos struggled as a passer for much of Saturday’s rain-soaked contest. Entering BC’s final drive, he was 7-for-13 for 75 yards.

But after Michigan State kicked a tie-breaking field goal to go up 19-16 with 4:12 remaining, Castellanos locked in, going 3-for-3 for 65 yards on the winning drive. All three completions went to Bond — BC’s leading receiver since the start of last season — with Castellanos overriding his teaching on the game-winner.

“I’ll probably get in trouble in film for throwing that because the defender was outside leverage on the chop route — a slot fade — so I wasn’t supposed to throw it,” the QB said, smiling. “But Lewis is a great football player. He won for me. I held the safety, and I just gave him a good ball and he made a good play.”

Bond, who finished with six catches for a game-high 102 yards, caught Castellanos’ pass at the 15-yard line and outran a final Michigan State defender to the goal line.

“He just trusts me,” Bond said. “If he sees 1-on-1 coverage like the last play, he’s going to look my way. So I know I’ve got to win for him, but most importantly for my receiver group and for the team. Because that’s a play I know I can make. I’ve just got to make it when it comes my way.”

Max Tucker’s end-zone interception 52 seconds later sealed a dramatic, gutsy win for BC. The sellout crowd celebrated by storming the field.

“It’s a dream come true,” Castellanos said. “I remember as a young kid just watching football games and the fans and students storming the field. That’s my first ever. It was amazing just to see how excited they were for us and for this team.”

A loss Saturday would have drained much of the positive momentum built during the first three weeks of the Bill O’Brien era, during which the Eagles upset Florida State in Tallahassee on national TV, throttled FCS Duquesne and then gave No. 6 Missouri a scare in a six-point road loss.

With the win — its first over a Big Ten team since 2019 — BC now sits at 3-1 on the season, with a home game against Western Kentucky up next. Of its eight remaining opponents, just one entered this weekend ranked in the AP Top 25 (vs. Louisville, Oct. 25).

Though still far from a national contender, the Eagles are well-positioned from a talent, coaching and schedule standpoint to compete in the wide-open ACC. At the very least, they’re relevant again after a decade-and-a-half of mediocrity, which Castellanos and his teammates are embracing.

“We came a long way,” Castellanos said. “This team has (come) a long way through the last couple of years, so it’s really amazing to see this program back up and running again, and it’s really fun to watch. I know the fans and everybody’s excited. Coach OB says all the time this program has got a buzz, and (Saturday) definitely showed that we have a buzz going on.”