Morneau jammed his left shoulder Sunday diving for a ball against the Detroit Tigers. The team said Tuesday he started experiencing headaches Monday and he was given a concussion test.
The team said he displayed "mild" symptoms during one part of the test, so it was decided to play it safe with a player who missed the second half of last season with a concussion.
The 30-year-old Morneau said he was feeling better Tuesday, but the headache hadn't completely subsided. He is expected to be tested again on Wednesday.