Upton, who has not played in a Major League exhibition game since a March 14 outfield collision with Desmond Jennings, said his main problem deals with his back "getting tight."
"Won't loosen up," he said. "Other than that nothing major is going on. I just think it's getting it loose and keeping it loose. I haven't been able to do that yet."
Earlier on Wednesday, Upton was asked whether his lower back felt good enough to play and Upton answered: "Maybe, but I don't think they're going to let me."
Manager Joe Maddon, Andrew Friedman, Rays executive vice president of baseball operations, and Upton sat down prior to the start of Wednesday night's game against the Pirates to discuss his situation. That's when they decided it would be best for Upton to start the season on the disabled list.
"We're going to have to [put him on the DL], we talked to him before the game," Maddon said. "The whole point is that in the beginning we thought it was going to be less time. Obviously, it is not. We've been forcing a finish line on this whole thing all the way through. So we took the finish line away and permit him and [head athletic trainer Ron Porterfield] to do their thing and for B.J. to get well without any kind of definitive ending in sight. We don't know what that means. Just get well, work on it ... See how that all plays out without a certain date."
In Upton's absence there are myriad candidates for the job, including Desmond Jennings, Ben Zobrist, Elliot Johnson and non-roster free agents Jeff Salazar and Jesus Feliciano.