Kowar will be called up to start Monday's game against the Angels, Josh Vernier of 610 Sports reports.
Kowar was dominant in the minors to begin the season, and he was named the Triple-A East pitcher of the month in May. Across six starts for Triple-A Omaha, the right-hander posted a 0.85 ERA, 0.90 WHIP and 41:10 K:BB in 31.2 innings. Kowar boasts a mid-90s fastball and an elite changeup and will fill in while Danny Duffy (forearm) is sidelined, though the 24-year-old could also make a case for a long-term rotation spot.