According to Field Yates of ESPN.com, Gresham has been kept off the field because he's recovering from hernia surgery.
Gresham was around for offseason workouts, but has not participated in any on-field drills. There is no timetable for his return.
"I will tell you if we were lining up to start training camp tomorrow, Leon would probably be lining up to start training camp as he did two years ago," Lewis said, via The Cincinnati Enquirer. "He's done incredible again."
Although Geno Atkins is expected to be held out of training camp, Lewis is confident that the All Pro defensive tackle will be recovered from ACL surgery in time for the season opener.