Butler exited Wednesday's Game 5 loss to the Houston Rockets with 2:33 remaining in the third quarter and did not return.
Butler, who had eight points, five rebounds and five assists in the 122-104 loss, injured the knee in a Feb. 23 loss to the Rockets. He underwent surgery to repair the meniscus in his knee and missed 17 games before returning to help the Timberwolves win the final three games of the regular season and earn the franchise's first playoff berth in 14 years.
"He had surgery, so it's a concern any time someone's hurt," Thibodeau said when asked if Butler's knee would be an issue in the offseason.
Butler was not asked about his knee during his postgame media availability, which he did in the Timberwolves' locker room during Thibodeau's news conference.
Butler declined to answer a question about the knee as he exited the locker room. Thibodeau said Butler was not bothered by the knee during the series until Game 5.
The Timberwolves were down 11 when Butler left the game, and they never cut the deficit to single digits.
"He just said it was sore," Thibodeau said. "We wanted to use caution."
Butler played the series with a sore right wrist, which the Timberwolves kept quiet after he injured it during his 31-point performance in the regular-season finale, a win over the Denver Nuggets that decided the Western Conference's final playoff berth.
A league source said Butler wasn't certain that he'd be able to play in Game 1 until his pregame shooting session a couple of hours before tipoff.
Butler, who averaged 15.8 points, 6.0 rebounds and 4.0 assists in the series, twisted his left ankle during the Timberwolves' Game 3 win. He refused to acknowledge that injury.
"It's all mental," Butler said after Game 3, in which he had 28 points. "At the end of the day, you tell your mind it doesn't hurt, and it doesn't hurt. You play through it."