Marcus Smart (calf) expects to return against Nets on Thursday

Marcus Smart has been sidelined since Jan. 30 with a calf injury.

BOSTON (AP) — Celtics guard Marcus Smart says he expects to be back on the floor for the first time in more than a month when Boston opens the second half of its schedule at Brooklyn on Thursday night (7:30 ET, TNT).

Smart said he felt good after practicing with the team Wednesday and that he has been cleared to return for the Celtics’ matchup with the Nets after missing the past 18 games with a torn left calf.

“As of right now I will be playing and attending the game tomorrow,” he said.

Coach Brad Stevens said Smart’s return should provide an instant lift.

“He’s a guy that knows how to play, plays both ends of the court and makes us better on both ends of the court,” Stevens said.

Boston would have had its full roster available for the first time all season Thursday, but Romeo Langford, who hasn’t played this season following wrist surgery, has been placed in the league’s health and safety protocol and won’t travel with the team to Brooklyn.

Stevens said Langford has been cleared to resume playing and would have been available if he hadn’t been placed in the protocol. Still, Stevens said he’s looking forward to the matchup with the Nets.

“It will be a great litmus test to see where we stand, because I think they are clearly the team that is going to be very difficult for teams to beat,” he said.

Smart has been the engine of the Celtics’ defense for most of his seven seasons in Boston and the team struggled on that end of the floor without him, going 9-9 and allowing opponents to shoot 46.5% and average 112.5 points over that stretch.

He said it’s been frustrating to watch that decline from the sideline with no ability to affect it.

“Especially when you know how good of a defensive team we can be — that we are,” Smart said. “And for me, being a defensive guy, it is tough. But being over there I have to look at it from a different perspective. When everybody else is looking at the negatives about it I’m looking at the positives.”

What he sees are things that can be fixed and that the Celtics can control with their effort.

“And what I have to do to bring back the defensive energy for us,” Smart said.

He’ll get his opportunity right out of the gate.

Brooklyn, which has the second-best record in the Eastern Conference, is averaging a league-best 121.1 points. Utah, which visits Boston on Tuesday, is third in the league with 116.6 points per contest.

Both teams have dynamic scorers. Smart will almost certainly spend time guarding them.

That could prove to be a challenging proposition considering that Smart’s first true chance to play 5-on-5 since his injury occurred during Tuesday’s practice session. It’s why he will have a minutes’ restriction for at least a few games, Stevens said.

But Smart said he isn’t too concerned about aggravating the injury and has been doing focused workouts to strength the muscle and guard against a recurrence.

“For me it’s always been like that, any type of injury that I’ve had,” Smart said. “When you go in thinking about it and worrying about it …. you can’t think like that. Because once you start thinking about that you start allowing that negativity to get in — that doubt — and things start going wrong for you.

“You have to trust your body. You have to trust the things that you’re doing, and just go out there and do it.”