Dizon, a versatile backup, injured his left knee during Saturday's exhibition loss at Pittsburgh, a game he started in place of Levy.
Both Dizon, a three-year veteran, and coach Jim Schwartz said Monday they did not know the exact nature of the injury, but that it would require season-ending surgery.
"That was big," linebacker Julian Peterson said. "He was one of those guys that could fill any spot on the field for us. He could play safety even if we were short on safeties."
Dizon, who had 19 tackles in 16 games last season, figured to be the primary backup on a group that lost two of last season's starters.
That leaves outside linebacker Peterson as the only returning starter. Levy is the projected starter in the middle while fellow second-year player Zack Follett takes the other outside spot.
Schwartz said losing Dizon will be tough.
"He was really good on all special teams and had played outside and inside," Schwartz said. "He's a guy we're going to have to work hard to replace."
There are a handful of veteran linebackers in camp, but few seem to have the potential to be much more than special teams players.
Eight-year veteran Vinny Ciurciu played with the first team most of Monday and Caleb Campbell, who played nine games as a rookie last season, have gotten significant time.
Landon Johnson and Ashlee Palmer also have seen several reps, but neither played a game last season.
Schwartz, a former defensive coordinator, said the team will keep searching for players to bolster not just the linebacking group, but the rest of the team.
"Probably all 32 teams aren't happy with their depth right now," he said. "We still have a lot of work to do to find guys we can rely on at the back end of the roster."
Levy had limited participation in Monday's practice but the coach didn't seem to think he would experience further setbacks. Levy's play as a rookie helped prompt the decision to trade Sims.
The Lions signed linebacker Lee Campbell, an undrafted rookie who went through offseason workouts with the team, to fill Dizon's spot on the roster.
Notes: S Ko Simpson came off the physically unable to perform list Monday. ... CB Jonathan Wade has a broken bone in his left hand and will practice in a cast. ... The team released DE Jason Hunter.
Copyright 2010 by The Associated Press