ATLANTA – The Atlanta Hawks learned Wednesday that forward Josh Smith has a strained tendon in his left knee, perhaps costing the team another key player for its playoff series with the Boston Celtics.

Smith is day to day and it’s not known if he’ll be able to go Friday night in Game 3. While the injury doesn’t sound as serious as some feared before he underwent an MRI, center Zaza Pachulia is also considered day to day because of a sprained left foot — and he hasn’t been on the court in nearly three weeks.

“I’m very concerned about it,” Coach Larry Drew said after a brief shootaround and film session. “I know Josh is a guy that if he’s just hurting, he’s going to play. He has that kind of toughness. But if it’s a situation where he can’t play, then he won’t be out there.”

The Celtics evened the series 1-1 on Tuesday night, rallying in the fourth quarter for an 87-80 victory. Smith, who has contended with tendinitis this season, appeared to be having trouble with his knee at several points. He finally headed to the locker room with 4:20 left and the Hawks’ offense bogged down without him in the post.

Now Atlanta is facing the prospect of trying to regain home-court advantage while missing its top two centers: Al Horford and Pachulia, and possibly Smith, who had taken over many of the offensive responsibilities in the lane after Horford went down early in the season with a torn pectoral muscle.

Smith averaged a career-best 18.8 points in the regular season. He led the Hawks to a Game 1 victory with 22 points and 18 rebounds, then had 16 points and 12 rebounds Tuesday.

“He’s just been so good for us,” Drew said. “He’s certainly a key component in order for us to have success. Obviously, we already have both centers out, and now we’re looking at our starting power forward possibly being injured. These situations have been happening to us all year. We’ve been snake-bit but we’re just hoping for the best.”

Horford wanted to return for the playoffs but already ruled himself out of the first round. Pachulia took over as the starting center and did a solid job, but was injured in mid-April and missed the last seven games of the regular season. The Hawks evaluate him on a daily basis, but he’s still wearing a protective boot on his foot and there’s no indication he’s close to returning.

“We’ve had a lot of bumps in the road this year, and we’ve been able to overcome them all,” said Jason Collins, who’s gone from third string to starting center. “This is just another one. There’s a lot of guys on this team with a lot of playoff experience. If someone goes down, someone else steps right in.”

But losing Smith would be huge. Ivan Johnson, a 28-year-old rookie and most likely candidate to step in at power forward, has been solid off the bench but isn’t likely to be nearly as effective as Smith over long stretches. The Hawks got a glimpse of how tough it would be without Smith when he left Game 2 with Boston clinging to a 74-72 lead.

Going to a small lineup and trying to spread the court, the Hawks couldn’t find any space to get open looks. They missed four of their next five shots, including a couple of long, desperate 3-pointers, and turned it over twice, allowing the Celtics to pull away.

“If you’re slow and methodical getting into sets, it feeds right into what they want to do,” Drew said. “We have to continue a mindset of trying to get out there and get easy fast-break points. If it’s not there, we have to run our sets and run them with pace. In that fourth quarter we just played too slow.”

But that’s not taking anything away from the Celtics, who evened the series without point guard Rajon Rondo, who was suspended for Game 2 after bumping an official in the opener. Paul Pierce scored 36 points to almost single-handedly beat the Hawks.

“We’ve won games this year without some of our key players,” Coach Doc Rivers said. “This is a tough-minded basketball team, and having a lot of veterans helps with that.”

Rondo will be back in the lineup when the series shifts to Boston for the next two games. Also, the Celtics still hope to get a contribution from shooting guard Ray Allen, who’s missed the first two games of the series with a sprained right ankle.