The problem? Tightness.
"Won't loosen up," he said (MLB.com). "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."
According to the MLB.com report, Upton met with manager Joe Maddon and executive vice president Andrew Friedman Wednesday night, and that's when it was determined to start the 2012 season on the DL.
Upton, 27, hit .243/.331/.429 with 23 homers, 81 RBI, 82 runs and 36 stolen bases last season. The Rays have the option of sliding Jennings to center with Upton out. They can then use Ben Zobrist in right field and play Jeff Keppinger at second base (with Sean Rodriguez at short). Elliot Johnson is also an option as a starter.
For more baseball news, rumors and analysis, follow @EyeOnBaseball on Twitter, subscribe to the RSS feed and "like" us on Facebook.