The Royals placed McArthur on the 15-day injured list Tuesday with a right elbow sprain.
A sprain indicates that he is dealing with ligament damage, although there's no word yet regarding the possibility of surgery. McArthur won't be eligible to return before the end of the season and also seems unlikely to be back for the postseason even if surgery isn't required. The former closer posted just a 6.87 ERA in the second half.