Phelps lasted a total of 8 2/3 innings in his two starts and allowed two runs on nine hits. He struck out eight and walked four. This season, the 25-year-old righty owns a 3.08 ERA, with 22 strikeouts and 11 walks in 26 1/3 innings. His only decision, a loss, came in his first start against Kansas City.
Manager Joe Girardi said that Phelps, who last pitched Wednesday, would likely be unavailable until Sunday.
The Yankees have 12 pitchers on their active roster ahead of Pettitte's return, but they have carried 13 arms before this season. Right-handed reliever Cody Eppley has already been both optioned to and recalled from Triple-A in 2012. A setback suffered earlier this week by Brett Gardner has allowed outfielder Dewayne Wise an opportunity to spend time with the big league club.