Rob Gronkowski to undergo back surgery

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nd Patriots star tight end Rob Gronkowski's back injury was worse than originally thought, and he will undergo surgery Friday to repair a ruptured disk, according to multiple reports.While there is no exact timetable for his return, Gronkowski is expected to miss eight weeks, according to the Buffalo News, which was first to report the story. On that timeline, he would be out through the AFC Championship if the Pats advance that far. MORE: Gronk's Patriots days are numbered | Week 13 Power RankingsIn an injury-hampered season, Gronkowski has just 25 catches for 540 yards and three touchdowns. He suffered the back injury and a punctured lung Nov. 13 on hit by Seahawks safety Earl Thomas. The Patriots will be forced to start Martellus Bennett, who also has been dealing with injuries this season. Bennett has 54 receptions for 540 yards and four touchdowns.