Allen was unavailable for the Colts' practice Thursday due to a hamstring injury, James Boyd of The Athletic reports.
Allen returned to practice July 31 after opening training camp on the active/PUP list with a calf strain, but he has now suffered another injury. Head coach Shane Steichen announced the injury to the media after Thursday's practice, outlining that the linebacker currently has no timetable to return. If Allen is forced to miss the team's preseason opener against the Patriots on Thursday, Austin Ajiake and West Weeks would be two of the top candidates to see an increase in workload at middle linebacker during the contest.