The three-time All-Star spoke before Portland's game against the Boston Celtics.
“I've definitely sprained my thumb a few times, but this was on a different level,” he said. “I couldn't even move it.”
The 6-foot-11 power forward had X-rays immediately following the injury but they were inconclusive. An MRI early Thursday revealed a radial collateral ligament tear.
Aldridge leads Portland with averages of 23.5 points and 10.4 rebounds and 1.18 blocks per game. He's one of just three NBA players averaging at least 23 points and 10 rebounds.
The injury is a big blow to the Blazers, who were already dealing with a depleted front line. Center Robin Lopez is out with a fractured right hand and center Joel Freeland is sidelined with a right shoulder strain.
Portland was 31-12 and sitting in second place in the Western Conference behind Golden State going into the game against the Celtics. But the Blazers had lost four of their last five.
“It's tough, we've been in a pretty good rhythm most of the season,” Aldridge said. “We've lost guys a lot this season — this is a totally different season from last year — but we have a lot of guys here who can step up and play better, and make plays. So I think the team shouldn't stall, we're going to get guys back soon, so that should be good for us.”
Aldridge said he will have surgery within the week. He was wearing a black sleeve on his wrist and thumb when he spoke to reporters in the Blazers' locker room.