"I think part of that is he has more confidence in himself, and he understands he can do it," Girardi said. "I think whenever you're a young player and you come up for the first time, something goes wrong and you're going to question a little bit. But I think he knows he has the ability to do it now."
Girardi said that he has been pleased by Warren's four-pitch mix and strike-throwing ability, as well as his ability to get left-handers and right-handers out and hold baserunners. Warren made 69 relief appearances for New York last year, going 3-6 with a 2.97 ERA.
"He does the little things, fields his position," Girardi said. "He just does a lot of things right."
Warren is scheduled to pitch the Yankees' fifth game of the season on April 11 against the Red Sox, and Girardi said that the Yankees plan to stay on rotation through their off-days.
Bryan Hoch is a reporter for MLB.com. Follow him on Twitter @bryanhoch and read his MLBlog, Bombers Beat. This story was not subject to the approval of Major League Baseball or its clubs.