As Caplan notes, the injury originally occurred during the Eagles vs. Buccaneers game in Week 6. Polk suffered the injury during the game and did not return. Polk also missed the Week 7 game against the Cowboys, which caused the team to cut Jordan Poyer and promote Matthew Tucker from the practice squad to the active roster.
Polk signed with the Eagles as an undrafted free agent in 2012. Many expected Polk to be drafted, but concerns with a lingering shoulder injury may have scared teams off. At this time, it's unclear whether the shoulder Polk had surgery on is the same one that was injured during his time at college.
In his second season, Polk largely played on special teams; recording 7 special teams tackles over the course of the season. That was tied for sixth most on the team. Polk made some contributions on offense despite only playing 47 snaps. Polk ran for 98 yards on 11 attempts and 3 TD. He also reeled in 4 receptions for 61 yards. Polk entered the season having lost a lot of weight and looked much faster and explosive than he did in his rookie year.
It's not uncommon to hear about NFL players fighting through the season with injuries and then having the necessary surgery required after the season is over. Polk will have a full off-season to recover in order to be ready to compete for a backup RB job in 2014 training camp.