ESPN.com earlier reported that the Lions said Delmas has surgery on his left knee but the team didn't specify which knee was operated on. However, Delmas hasn't practiced for most of the past 10 days because of soreness in his left knee.
Delmas again was not at practice Wednesday.
"As soon as he is able to get out and go and keep it from getting sore, then he'll be back," Lions coach Jim Schwartz said Wednesday. "The reason he ended up getting it done was to get him back on the field. First it was sore, then he tried to go again, and we just weren't getting anywhere with it."
The team didn't specify the nature or severity of the surgery but said it was performed by Dr. James Andrews.
"We really don't have any timetable," Schwartz said. "He's not day-to-day, he's more week-to-week."
Delmas, 25, was a Pro Bowl alternate last season despite missing five straight games due to a sprained MCL in his right knee. He had 51 tackles and five passes defensed in 11 games.
Without Delmas, the Lions allowed opponents to throw for 300 or more yards in three of their final five regular-season games, including Matt Flynn's 480-yard, six-touchdown performance in the last weekend. It was almost as bad in the opening round of the playoffs, when New Orleans' Drew Brees passed for 459 yards and three scores.
Amari Spievey, the other returning starting safety, said losing Delmas late last year hurt.
"He has a big impact on our defense. It was kind of difficult to get used to it without him in there," he said.
Spievey and nine-year veteran Erik Coleman, who played just four games last season, are the likely starters when the Lions face Cleveland in Friday's preseason opener.
Information from The Associated Press was used in this report.