It's either a knee or ankle injury.
Oldenburg, who was a fifth round pick of New England last year, was competing for a reserve role at both guard and tackle.
In other injury news, the ankle injury suffered by tight end Dustin Keller in Saturday's scrimmage turned out to be very minor. He participated in both Monday practices.
Add rookie wide receiver Marcus Henry to the injury list. He sat out today with a leg injury of some kind.