BOSTON (CBS) — While at an event at the Naismith Basketball Hall of Fame on Friday, Brad Stevens was hit with a simple (yet extremely complex) question from a young Celtics fan.

“Why did you trade Isaiah Thomas?” the young man asked.

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The young boy’s question drew a round of applause from the crowd, and got a lengthy answer out of the Celtics head coach.

“That’s an excellent question and here is what I’ll tell you: What he did in Boston the last two and a half years was incredible,” answered Stevens. “I said earlier what he has meant to me, what I think of him, how great he has been in the locker room and what kind of teammates he’s been. Those are all really, really, really hard decisions. That’s the hard part of being a professional basketball coach. I’ve stared at the wall many times and thought, ‘man, having guys in college for four years and knowing when they’re coming and going is a pretty good way of doing things.’ That’s a tough one.”

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Thomas was sent to the Cleveland Cavaliers along with Jae Crowder, Ante Zizic and the 2018 unprotected Brooklyn Nets pick for star point guard Kyrie Irving on Tuesday night. While Irving is a superstar talent in the NBA, Thomas was the face of the Celtics franchise for the last two-plus seasons, quickly becoming a favorite in Boston for his offensive firepower and give-it-all attitude.

“He and Jae both, I feel blessed to have had a chance to coach them,” Stevens said (via Mass Live’s Tom Westerholm). “I learned a lot from them. I think the players in the locker room would echo the same.”

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There’s no doubt he’s going to be missed by Celtics fans — and his coach.