Patriots tight end Rob Gronkowski has definitively known for two weeks that he would need back surgery this summer. Now, he can begin preparing himself for his latest trip to the operating room. Gronkowski will undergo back surgery Tuesday, according to reports. Dr. Robert Watkins will conduct the procedure in California. Watkins operated on Gronkowski’s back in 2009, recently repaired Giants defensive end Jason Pierre-Paul’s back and is considered one of the best in the field. The Herald has reported since last month that back surgery would keep Gronkowski out until late September. However, it’s possible the timeline could fluctuate depending on Gronkowski’s rehabilitation program. The Patriots completed their nine-week offseason workout program this week, and Bill Belichick said every player was present for mandatory minicamp, including Gronkowski. Though the tight end didn’t practice, his implied presence means he was rehabbing his injuries with the team’s medical staff. The back procedure will be Gronkowski’s sixth surgery since February 2012, including one on his ankle and four on his forearm. The Patriots would seemingly place Gronkowski on the physically unable to perform list before the start of training camp to protect themselves if he isn’t ready for the regular season. That way, he can be removed from the PUP list at any point during training camp. If Gronkowski remains on the PUP list at the start of the regular season, he would be out for a minimum of the first six games. Players cannot be placed on the PUP list if they’re already on the roster once camp has started.