Castillo (hamstring) is making good progress and has thrown off a mound, but he's not expected to return from the 15-day injured list for the Mariners' three-game series in Houston that begins Monday, Shannon Drayer of SeattleSports.com reports.
Seattle has confirmed Bryce Miller as its starter for Monday's series opener in Houston, and Logan Gilbert and George Kirby are considered "most likely" to go Tuesday and Wednesday, respectively. That means Castillo likely won't return from the IL when first eligible, but he's still very much in the mix to make a final regular-season start against Oakland later in the week. Castillo currently sits with a 3.64 ERA and 1.17 WHIP in 175.1 innings this season.