Smart, who suffered a torn left oblique four weeks and a day ago after running into Orlando Magic center Nikola Vucevic's left hip, had said Sunday he was "hopeful" he would return after going through practice for the first time since the injury occurred.
"We're taking it one day at a time but we're definitely in the right position, right time, progressive phase to where I'm getting really close to coming back here," Smart said after taking part in Boston's practice.
Smart's timeline shifted from "really close" to "arrived" late Monday afternoon, when the Celtics announced shortly before Celtics coach Brad Stevens' pregame presser that Smart had been upgraded from questionable to available for Monday's game.
Stevens said before the game he would ease Smart back into the rotation, and only played him 15 minutes in Game 4. Smart ended the game 1-for-7 from 3 with two assists and a steal.