Already without an injured Garnett for possibly the remainder of the playoffs, the Celtics have lost backup forward Powe for the duration because of a torn anterior cruciate ligament and meniscus in his left knee.
"That is just a tough injury for a kid who has done everything right," coach Doc Rivers said after practice on Tuesday.
Powe was injured in Boston's 118-115 Game 2 victory over Chicago on Monday. The injury is unrelated to the right knee sprain that sidelined him in March for almost a month.
"He's devastated. He worked so hard, especially coming back from his recent injury and in establishing himself as a player in this league. He is taking the news hard, but he is a good kid, and he knows he'll be back at it soon," Powe's agent, Aaron Goodwin, told ESPN.com's Chris Sheridan.
Goodwin told Sheridan there have not yet been any discussions on when surgery might take place, and a second opinion will be sought first. Goodwin estimated the recovery time for Powe at six to eight months.
Powe did not return to the bench Monday after going to the locker room early in the second quarter. Boston's last-second win, courtesy of a Ray Allen 3-pointer, evened the first-round series against the Bulls at a game apiece.
"I feel bad for him," Bulls guard Ben Gordon said. "Anytime somebody has that severe an injury, that's every athlete's worst nightmare. Our best wishes go out to him, but they still have other big guys."
Garnett, the centerpiece of Boston's unprecedented 17th NBA title and a key to its hopes of trying to repeat, has been sidelined by a knee injury that limited him to four games over the final two months of the season.
Celtics forward Brian Scalabrine, who is recovering from concussions and has played just 60 minutes since Jan. 25, has been cleared to practice on Wednesday. Point guard Rajon Rondo is nursing a sore ankle and won't practice, but Rivers said he will play in Game 3 on Thursday in Chicago.
"We have all hands on deck now," Rivers said.
Rivers said Powe hurt himself on a spin move but stayed on the court trying to play.
"He played 3 minutes with a torn ACL," Rivers said. "That says so much about Leon Powe the person."
Powe averaged 7.7 points and 4.9 rebounds per game this season, missing 13 straight games down the stretch with a different injury in his right knee. He returned for the last two games of the regular season and was expected to have increased responsibilities because of Garnett's absence.
"Obviously, with Garnett out and Powe out, that's to our advantage," Bulls coach Vinny Del Negro said, "but you hate to see guys get injured."
Information from ESPN.com's Chris Sheridan and The Associated Press was used in this report.