"I personally feel that we had to do something that is nonconventional, if you will, and if this doesn't work out, then we'll work on something else," Tracy said.
Tracy notified his starters and Guthrie of his decision before Tuesday's series opener against the Phillies, and he acknowledged the move to go to a four-man rotation is "a little bit outside the box."
The Rockies' rotation has a Major League-worst 6.28 ERA in 338 innings, the second fewest in baseball, and Tracy is concerned about the abundance of work the bullpen was getting.
Tracy hopes that moving Guthrie, who has a 13.50 ERA in his last three starts, will help him get back on track. Guthrie was the Opening Day starter this season, and his name has recently appeared in trade rumors.
"He understood [the move to bump him to the bullpen] and actually came out and said that he'd be ready to pitch tomorrow," Tracy said. "He's not feeling good about where he's at as far as how he's pitching, not the fact that we've had to move him to the bullpen. He's not performing right now."