The Eagles' already-suspect linebacker depth has taken another big blow. 

Backup middle linebacker Joe Walker tore his ACL during Thursday night's preseason game in Pittsburgh and is out for the season, a league source confirmed to CSNPhilly.com.

The Philadelphia Daily News first reported that Walker was out for the year.  

Walker was a seventh-round selection out of Oregon, but was slated as the Eagles' backup middle linebacker behind the oft-injured Jordan Hicks. 

Despite coming from a two-gap, wait-and-react defense at Oregon, Walker took to Jim Schwartz's scheme quickly and asserted himself as a top backup. Now, he'll have to sit out his entire rookie season after a promising start to the preseason. 

After Walker came out of the game on Thursday, undrafted rookie Don Cherry from Villanova took his spot. With Walker done for the year, the Eagles are left with Cherry, Najee Goode, Deontae Skinner, Quentin Gause and Myke Tavarres as the remaining depth pieces at the position. 

This injury makes it much more likely the team will be looking for another backup linebacker.