Scalabrine, who has been an integral part of the Celtics this season, was hit during Tuesday's practice by Patrick O'Bryant while going up for a rebound.
"He got hit in the head and he just had a mild concussion," coach Doc Rivers said. "I don't know if he was out but he was close. He'll be out probably for quite some time because of that, would be my guess, for quite some time."
Rivers said Scalabrine is expected to be out for at least a week. "I don't know that but two concussions in [three] days can't be good for you," he said.
Scalabrine was also hit in the head in the second half of Sunday's blowout win over the Dallas Mavericks.
Ray Allen said he saw Scalabrine get hit and fall to the ground hard on Tuesday.
"It just looked like he got hit in the head," Allen said. "I know he got hit last game in the head. He went down and just stayed down and looked disoriented."
Scalabrine has started six games this season at center, filling in for Kendrick Perkins, who was nursing a sore left shoulder.
Rivers said that because of Scalabrine's injury, Tony Allen will likely make his return after missing the last 11 games with a right ankle sprain.