CHICAGO — Boston Celtics star guard Kyrie Irving sat out Monday night's game against the Chicago Bulls because of a bruised left quadriceps.

Acquired for Isaiah Thomas in a huge off-season trade with Cleveland, Irving is averaging a team-leading 23.7 points. The four-time All-Star took a knee to the quad in Sunday's win at Detroit and finished with 16 points on just 4-for-12 shooting.

Irving missed his second game of the season. Coach Brad Stevens said doesn't expect him to be out long, though he added: "Who knows with muscular injuries."

Stevens also said doctors will re-evaluate forward Marcus Morris' ailing left knee on Tuesday. Morris sat out for the second straight night and the third time in four games. He also missed the first eight games of the season.

Forward Al Horford said he was ready to play after banging his right knee with Detroit's Anthony Tolliver.

Bulls leading scorer Lauri Markkanen also missed Monday's game because of back spasms. He participated in the morning shootaround. Coach Fred Hoiberg said Markkanen got treatment afterward, but his back continued to tighten.

The promising rookie also had some back issues before the season.

"We're taking the cautious approach with it," Hoiberg said. "It has not been a big issue. We just had the one episode with the back. He's got a little bit of soreness in there now and, again, we're hoping to get him back into the lineup soon."

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