Moustakas said he will seek a second opinion on the injury.
"We're still talking with the doctor, get a second opinion, see where we're at,'' Moustakas said. "You can't rule anything out. I just want to go out there and be healthy and get prepared to play baseball again. That's where I'm at right now."
The Royals had activated Moustakas on Saturday from the 15-day disabled list, after he had recovered from a small fracture in his left thumb.
"It's disappointing. It's just one of those things that happen in sports," Moustakas said. "You can't control everything. Me and Gordo [Alex Gordon] are going out trying to make a play and unfortunately we both got hurt. That's just part of sports."
Moustakas is batting .240 with seven home runs and 13 RBIs this season.
The Associated Press contributed to this report