WALTHAM, Mass. — Doc Rivers knew the O’Neal veterans – Shaquille and Jermaine – would miss time with injuries after the Celtics signed them to plug the hole left by another ailment.

Boston’s coach just didn’t think the two free agent centers would miss as much time as they have.

Shaquille O’Neal sat out practice on Monday and will be sidelined for the second straight game with a sore right hip when the Celtics try to bounce back from a loss to the lowly Washington Wizards with a win tonight against the team with the worst record in the NBA, the Cleveland Cavaliers.

“It’s not a serious injury,” Rivers said, “but he’s also 38 years old and we’re going to be very cautious.”

The Celtics start a four-game West Coast trip in Portland on Thursday night, and Rivers said he thinks O’Neal will play in at least one or two of those games.

Both O’Neals were signed in the offseason as free agents to fill in for Kendrick Perkins, who hurt his knee in Game 6 of the NBA finals and missed the Lakers’ title-clinching 83-79 win in Game 7. Perkins has targeted the game against the Dallas Mavericks on Feb. 4 for his return.

“He looks great,” Rivers said after Monday’s practice. “He went through the entire practice because we needed him.”

That’s because the O’Neals didn’t participate.

Shaquille has played in 33 of the Celtics’ 43 games, averaging 9.9 points, 4.9 rebounds and 21.3 minutes per game.

Jermaine, 32, has played in only 17 games with averages of 5.2 points, 3.8 rebounds and 18 minutes. He sat out 19 games, then played in the next 10 before missing the last six with a knee injury. But he’s out for four weeks, hoping rest and rehabilitation will get him back on the court.

Rivers said he didn’t know when the team signed them how much they’d be able to play.

“We knew with Shaq and J.O., we would have injuries,” he said. “The history told you that. We just didn’t know it would be this amount right now, but we’ll get through it.”

Despite their absence, the Celtics are 33-10, tops in the Eastern Conference. But for now, their healthy centers are rookie Semih Erden and Glen Davis, normally a forward. Davis has played all 43 games with nine starts and is averaging 30.8 minutes per game. He’s played more than that in 11 of his last 16 games.

“I feel like I can be in better shape,” he said. “I need to be a guy who’s in shape for 48 minutes instead of 35. Those little minutes count.”

Davis prefers to come off the bench, and Erden did start Saturday night’s game when Shaquille O’Neal was sidelined. The Celtics led by 16 in the first half, then began settling for outside shots, giving the Wizards a chance to come back. It didn’t help the Celtics that they scored just 11 in the fourth quarter, a season-low for any period. They missed 18 of 23 shots in that quarter.

They certainly shouldn’t lose tonight. But they’ve already lost to the Cavaliers, 95-87, in Cleveland. Despite the results, though, Rivers said the Celtics don’t overlook weak opponents.

“If we do, then we’ll lose,” he said. “I don’t think our team does that. I just think we come out and we assume wins and sometimes they don’t happen. You have to go out and earn them.”