The Twins put All-Star catcher Joe Mauer on the DL yesterday while he reportedly deals with a condition known as bi-lateral leg weakness. The injury not only affects the strength of his legs, but also is reportedly affecting his shoulders and hips. According to that post from SB Nation's Baseball Nation, Mauer will head to Baltimore on Friday to visit a specialist, and we should have more information on the injury and potential timetable for return after we learn the results of that consultation.
One of the most common causes, as noted by leo3375, is influenza. Since Mauer currently has the flu it's easy to lean on it as the cause, but we'll still need to wait until we have more information later today.
Michael Rand of the Minneapolis Star Tribune is using this latest injury to raise questions about whether or not Mauer should be playing catcher on a full time basis. He does not think that this injury will be the end of his career at the position, but does wonder whether or not it is worth it to put your franchise player at the position that causes the most stress on your body when he has the athleticism and talent to succeed at other places on the diamond.