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Celtics’ Theis likely out for season, while Smart sidelined indefinitely

Daniel Theis was averaging 5.3 points per game this season.Elsa/Getty

Before the Celtics held a brief practice Monday, coach Brad Stevens gathered his team — at least those who were healthy enough to be there — and offered a quick message.

“This is what you guys have been doing this all year for,” he said, “because we’re going to count on all of you.”

The Celtics have dealt with myriad injuries and maladies throughout this season, but on Monday they were dealt even more blows.

Backup center Daniel Theis, who went from an unknown to a revelation during his rookie season, was diagnosed with a torn meniscus in his knee. He is considering various surgery options and is expected to miss the rest of this season.


Also, guard Marcus Smart sprained his right thumb when diving for a loose ball in a loss to the Pacers Sunday. He is undergoing further tests, but is expected to be sidelined for an undetermined amount of time.

The Celtics are already without guard Jaylen Brown, who is sidelined with a concussion he suffered after taking a hard fall in a win over the Timberwolves last Thursday. Also, point guard Kyrie Irving left Sunday’s game with a sore left knee and did not practice Monday, and it appears increasingly likely that he will be rested for at least a few days.

“I would assume we’re going to tread slowly,” Stevens said.

Forward Al Horford remained at home Monday as he recovers from an illness, so it is unclear what kind of roster will be available to Stevens when the Celtics face the Wizards on Wednesday, but it will be an undermanned one.

The good news for the Celtics is that their position in the Eastern Conference standings is all but locked in. They are in second place, 3½ games behind the Raptors. Their already slim hopes of catching Toronto have probably been dashed, but they remain seven games ahead of the third-place Pacers. With just 15 games remaining, that is almost certainly too large of a gap to overcome, even with Boston’s injuries.


Nevertheless, this sudden surge of trouble is concerning for a team that seemed to find a rhythm after the All-Star break when it had its full complement of players, aside from the injured Gordon Hayward.

“We are equipped and we will battle on I think is the way you’ve got to look at it,” Stevens said. “I’ve never been in a season that’s been like this.”

Stevens said it was unlikely that the Celtics would add a player immediately, mostly because they no longer have an empty roster spot so would have to waive a player to do so. But there is one clause that could allow Boston to sign a player with an injury exception.

If three players miss three games in a row concurrently, and a league official determines it is likely that those three players and a fourth player are likely to miss 10 more days, Boston could add an extra player. This would be an unlikely confluence of events, but given the Celtics’ recent string of maladies, it’s certainly possible.

The Celtics have already recalled forward Guerschon Yabusele from the Maine Red Claws, and guard Kadeem Allen, who is on a two-way contract, has been with the team since the weekend.

The Celtics thought Theis had a knee-to-knee collision during the loss to the Pacers Sunday night but later realized that his leg buckled backward. Theis, who signed a two-year deal last summer after playing professionally in Germany, was averaging 5.3 points and 4.3 rebounds in 14.9 minutes per game. His greatest value may have been his skill and versatility as a defender.


“Just how much effort he plays with — he’s like a sponge,” point guard Terry Rozier said. “He learned so quickly coming from playing in Europe and then coming here and playing. I know it’s not the easiest thing, but he made the best of it.”

The Celtics do have some reinforcements, however. The team filled its final roster spot Feb. 8 by signing free agent big man Greg Monroe, and he will see an expanded role in Theis’s absence.

After a quiet start, Monroe has found a rhythm on offense recently. Over the past five games, he is averaging 10.4 points while shooting 64.7 percent from the field.

While Theis is a long, agile, and versatile defender, Monroe is more of a skilled scorer who plays below the rim. As such, Stevens acknowledged that the Celtics might alter their approach a bit in the coming weeks. But he said the team still has some flexibility.

“One of the reasons why we wanted to sign Greg was because you never know what can happen with your depth inside,” Stevens said. “But then there are going to be times where we’ve played smaller with Theis at the 5, and he’s been able to switch and do some of those types of things. So you might see Semi [Ojeleye] there a little bit.”


Irving’s sore knee, meanwhile, is not believed to be a long-term issue. But the Celtics will be overly cautious in the coming days.

“We’ve said if we need to rest or the knee becomes an issue, then automatically everything stops and we go from there,” Stevens said. “What we’re really focused on now is making sure that knee feels as good as possible heading into April. I don’t know what that means from the standpoint of playing this week, next week, any of that stuff.”

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Adam Himmelsbach can be reached at adam.himmelsbach@globe.com. Follow him on Twitter @adamhimmelsbach.