The White Sox placed Castillo on the 15-day injured list Thursday with a right shoulder impingement.
Castillo posted a 7.52 ERA in six starts and then a 13.50 ERA in three relief outings after being demoted to the Chicago bullpen. The veteran right-hander will be eligible for activation during the ALDS, but considering how he's pitched, there's a good chance Castillo's season is over even if he's healthy.