According to Thursday's St. Petersburg Times, Upton has a torn labrum in his left shoulder that limits his swing, and it will require the 24-year-old to undergo offseason surgery.
"At this point, there's nothing I can do about it, especially with Carl [Crawford] and Evan [Longoria] already out," Upton told the Times. "It's a little too late in the year, especially with the position we're in, for me to pull up lame. I just have to suck it up and keep grinding, and do what I can do to get on base and get in scoring position for these guys."
Maddon acknowledged on Wednesday that Upton has been excused from pregame batting practice, and the skipper said he had "no problem" with the outfielder saving his swings for the game. The decision dates back to a mid-August series in Texas in which Tampa Bay did not take on-field BP for three straight days and Upton reported that his shoulder felt better.
"By not swinging as often, I think his swings in the game are better," Maddon said.
And while Upton's productivity at the plate has improved -- he had three straight multihit games starting Aug. 28 -- his power has suffered.
Upton entered Thursday's game with a .397 slugging percentage and eight home runs, compared to last season's .508 slugging percentage with 24 homers.
Although Maddon can't be sure what's going on inside Upton's body, the skipper said his shoulder has "probably" impacted Upton's ability to drive the ball at the plate.
According to Upton's comments in the Times, it definitely has.
"If I could swing the way I wanted to, there would be less doubles and more homers," he said. "A lot of my doubles have been off the wall or they one-hop the wall. If I could have really put a full swing into it, I think the story would be a little different."
For now, Upton has found other ways to get on base with a .381 on-base percentage and team-leading 88 walks. And with the Rays already hit hard by injuries to Longoria (right wrist fracture) and Crawford (subluxation of right middle finger tendon), Tampa Bay has no intention of Upton joining them.
Maddon confirmed that playing Upton won't do any further damage to his shoulder, and with the Rays in the middle of a franchise-first playoff race, he will remain as the starting center fielder.
Brittany Ghiroli is an associate reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.