The school said Ridley will be out indefinitely, though coach Shaka Smart said he hopes to have Ridley back this season.
The 6-foot-9 Ridley, a highly touted recruit coming out of high school, hadn't quite lived up to expectations from when he was brought into the program by former coach Rick Barnes.
However, Ridley is averaging career highs of 12.7 points, 10 rebounds and 3.4 blocks through the first 11 games of the season under Smart.
Lammert averages 5.9 points and 5.1 rebounds per game, more than the other two big men.
"We're not the only team in America that has to deal with a curveball like that," Smart said "I told our guys, we're not going to feel sorry for ourselves. They don't need to reinvent the wheel. They just need to do the things that we've asked them to do all along a little bit better."
One potential adjustment is to induce a faster pace with a smaller lineup, including the full-court pressure defense that some of Smart's teams at VCU were known for before he came to Texas this season.
Information from The Associated Press contributed to this report.