PHILADELPHIA -- For nearly a week, Jacob deGrom has battled an illness that he recently discovered is a bout of strep throat. deGrom began taking antibiotics this week and departed Philadelphia feeling better. But as a precaution, the Mets are pushing back his next start from Friday to Saturday.

Jason Vargas will shift back into his original rotation spot, starting Friday in St. Louis.

The Mets initially swapped those two pitchers as a way to ensure deGrom would start against the division rival Phillies next week at Citi Field, instead of against the Brewers. That is no longer possible with deGrom shifting to Saturday.

Still, deGrom’s health is paramount. The reigning National League Cy Young Award winner was bothered by illness during his last start, in which he allowed three runs in five innings in Atlanta. He has allowed nine runs in his last two starts, spanning nine innings.

If deGrom’s illness lingers longer than the Mets expect, they can use either Steven Matz or Noah Syndergaard on Saturday in his place. Matz would be on three days’ rest, but threw just 31 pitches in his last start, while Syndergaard would be on regular rest because of Thursday’s off-day. But deGrom fully expects to be ready by Saturday.

The move marks another adjustment to Vargas’ schedule in what has been a theme of the left-hander’s tenure with the Mets. Pitching on 11 days’ rest in his last start, Vargas allowed four runs and recorded merely one out. The next day, both manager Mickey Callaway and general manager Brodie Van Wagenen offered Vargas votes of confidence as a member of the Mets’ rotation.