From ESPN.com:

Igawa was optioned to Class-A Tampa, keeping right-hander Darrell Rasner in the big leagues after he pitched 5 2/3 shutout innings Sunday in a victory over the Seattle Mariners. New York had planned to send Rasner right back to Triple-A Scranton/Wilkes-Barre after his start, but changed course on Monday. ...

"There's some things he's got to fix, mechanically, we believe," Yankees general manager Brian Cashman said. "He's got major league ability, we have no doubt about that. We've seen it in three games. But we haven't seen it consistently."