The illness that has sidelined Celtics guard Devin Carter since early in the preseason is mononucleosis, according to a source.
Carter has been out since Boston’s preseason opener on Oct. 8, during which he scored 17 points on 5-of-9 shooting in a win over the Cleveland Cavaliers. He missed the final three exhibition games and has been away from practice.
The team is unsure when Carter will be healthy enough to return. Mono cases can take several weeks or more to recover from.
Despite his prolonged absence, the 24-year-old successfully parlayed his Exhibit 10 tryout contract [into a two-way deal](https://www.bostonherald.com/2026/10/17/ex-lottery-pick-reportedly-earns-two-way-celtics-contract-after-tryout/) with Boston. Carter, who was traded from Sacramento to Atlanta this offseason and then waived, can be active for up to 50 Celtics games this season.
“Obviously, he’s very talented,” Celtics head coach Joe Mazzulla said Sunday. “He’s done some good things in the league and been there. But with us, he’s just kind of put his head down. (He was) really impactful on the defensive end of the floor in the Cleveland game. He did some things for us offensively. But I think we have a guy who came in with a great attitude, did what we asked him to do.
“Obviously, you see the talent and the impact that he could have, and you want to be able to invest in that.”
As a recent lottery pick (13th overall in 2024), Carter is a young, high-upside addition to the Celtics’ veteran backcourt. Top guards Payton Pritchard and Derrick White are 28 and 32, respectively, and reserve Mike Conley is one of the NBA’s oldest players at 39.
Carter’s pedigree — including a decorated collegiate career at Providence College — made him the pre-camp favorite to land Boston’s third and final open two-way spot.
But Mazzulla said it was difficult to part with the Celtics’ other four tryout players (Milos Uzan, Hayden Gary, Gabe McGlothan and Tucker DeVries), especially after watching them start Friday’s preseason finale in Philadelphia against Jaylen Brown, LeBron James, Tyrese Maxey and Joel Embiid and [lead the C’s to an upset victory](https://www.bostonherald.com/2026/10/16/celtics-deep-reserves-cap-preseason-with-stunning-ot-win-over-76ers-four-takeaways/).
“It’s always tough in those moments because the group we started the other night were fighting for that (spot), as well,” Mazzulla said. “I thought they worked their asses off from really Summer League and August, September, and you can only pick one of them. We always talk about how the bigger guys impact the culture in the locker room, but I thought the team that started the other night — the Exhibit 10s, the two-ways, the guys fighting for the two-ways — those guys are just as important and really helped us have a great training camp. And Devin was part of that, as well.”