The move comes as no surprise, as Lyles started Thursday for Triple-A Oklahoma City and was removed after just two innings.
It will be the fourth start of the season for the 21-year-old right-hander. He's 0-1 with a 5.29 ERA. In his most recent start, Lyles allowed six earned runs in five innings against Texas.
Earlier on Sunday, Mills confirmed that lefty ace Wandy Rodriguez (4-4, 2.14 ERA) would get the ball in Game 1 of the twin bill against the Rockies' Juan Nicasio. Lyles will face Alex White in the nightcap.
A new rule this season allows for a 26-man roster on doubleheaders scheduled more than one day in advance. That means Lyles' callup will be only temporary.
"I'd like to have the 26-man rule all the time," Mills kidded. "But yeah, it's real good, definitely for a doubleheader like this."
Still, it might not be long before Lyles is back again. The club will eventually have to make a decision on a starter for their game next Sunday against Cincinnati, which would fall on Lyles' turn in the rotation.