With Baker having second thoughts about his scheduled start on Monday afternoon, the Twins chose to put him on the 15-day DL, giving the right-hander a few more days to rest his strained right elbow.
"He was really unsure whether he was going to be able to go in this start," Twins manager Ron Gardenhire said. "He thought he could pitch, but he didn't know the ramifications after he pitched, how that would come into play, if he'd be really sore and end up having to miss starts after it."
In place of Baker, Anthony Swarzak will start the first game of Monday's doubleheader against the Indians. He was originally scheduled to pitch the nightcap of the twin bill.
Diamond, a lefty who is 4-8 with a 4.70 ERA for Triple-A Rochester this season, will make his Major League debut in Game 2 of the doubleheader.
A 2010 Rule 5 Draft selection by the Twins, Diamond will take the 40-man roster spot once occupied by Eric Hacker.
"We'll give [Diamond] a chance to kind of regroup, sit around here, see what it's all about, get a feel for a few more things," Gardenhire said. "We're facing the Cleveland Indians and he's being thrown into a fire here, so we'll see how he does. He's a nice-looking young pitcher."
Baker was scheduled to make his first start since July 5, when he exited the game after just five innings due to a right elbow strain.
Instead, he'll be on the DL until at least Friday with the move retroactive to July 7.
"He wanted to pitch, but it just wasn't going to work out," Gardenhire said. "He was very unsure of how it was going to turn out. You can't have uncertainty out there."