The Angels placed Klassen on the 15-day injured list Sunday with left low back inflammation.
The 24-year-old rookie sports a 2-2 record, 3.67 ERA, 1.63 WHIP and 24:21 K:BB over 27 innings this season. The Halos recalled Mitch Farris from Triple-A Salt Lake to provide bullpen depth during Klassen's absence.