Monroe, who originally agreed to sign with Boston last week after securing a buyout agreement with the Phoenix Suns, had to wait while the Celtics navigated the trade deadline before being brought on board to take Boston's final roster spot.
Boston did not make any moves at Thursday's deadline, electing to sit on its treasure trove of draft assets rather than splurge on a bench upgrade.
Monroe met the Celtics in Washington on Wednesday. The Georgetown product was photographed working out in Celtics gear on the school's campus before Boston players engaged in optional off-day work.
Monroe said before Thursday's game against the Wizards that he chose Boston because he thought he could fill a role on a contending team.
"Once the buyout was done, it was just about seeing where I would be most comfortable, having the best opportunity," Monroe said. "Obviously these guys have been playing well all season. It was about joining them and trying to help them as much as possible. Just try to get comfortable with these guys and try to keep this thing going."
Monroe was wearing No. 55 when he debuted against Washington.
Sources previously told ESPN that Monroe would ink a one-year, $5 million contract with the Celtics. Boston utilized the disabled player exception from Gordon Hayward's ankle injury to sign Monroe.
Monroe averaged 11.3 points and 8.0 rebounds during 20 appearances for the Suns this season. The 6-foot-11 big man, the seventh overall pick of the Pistons in the 2010 NBA draft, was traded to Phoenix in November as part of a package in which the Bucks brought back Eric Bledsoe.
Monroe admitted he considered his hometown New Orleans Pelicans before deciding to join Boston.
"As soon as the buyout was open, I was analyzing all my options," Monroe said. "Obviously, I want to wish a speedy recovery to DeMarcus [Cousins]. He went down and obviously they had a need there at my position. I definitely analyzed going there. It's home for me, so it was definitely a tough decision, but it was hard to pick against here."
The Celtics, who have struggled to consistently generate second-unit offense, are expected to initially deploy Monroe off the bench while coach Brad Stevens figures out how he can maximize Monroe's skill set. While the Celtics often like to stretch the floor with 3-point shooting bigs, Monroe's passing skills should help him integrate with various units, and Boston can lean on his post-up skills on a team that lacks back-to-the-basket threats.
In eight NBA seasons, Monroe has averaged 13.9 points and 8.7 rebounds. While the Celtics have been largely mum on his addition, rivals have suggested Monroe is a great pickup for a team that sits atop the Eastern Conference.
"He's an excellent passer. He's a low-post scorer. You have to bang with him down on the block. And [Monroe will] fit into their system as a passer," Raptors coach Dwane Casey said. "You can still space out [Al] Horford and play [Monroe] down low. And [Monroe] is a very, very underrated passer. I think that's something that, with their movement away from the ball and the splits that they have and the [dribble handoffs] that they have, another ballhandling big gives them another weapon to go with what they already have."