Castillo (hamstring) will undergo an MRI on his left hamstring on Monday in Seattle, Daniel Kramer of MLB.com reports.
Castillo left Sunday's start against the Cardinals as a result of a left hamstring strain. He will now receive additional imaging on the leg to determine the severity of the injury. Manager Dan Wilson did not speculate if he believed that Castillo will be able to make his next scheduled start or not, but more information on Castillo's status will come Monday.