ARLINGTON -- Brandon Webb will undergo an MRI on his right shoulder on Tuesday. Webb, who has been on the disabled list since Spring Training, was scratched from a medical rehabilitation assignment on Friday when his shoulder tightened up. "This is certainly a setback and an unfortunate one," Rangers assistant general manager Thad Levine said. "We'll know the extent of it on Tuesday." Webb hasn't pitched in the Major Leagues since Opening Day of 2009 with the Arizona Diamondbacks. He was the 2006 National League Cy Young award winner and a 22-game winner in 2008 but has been sidelined for over two years with shoulder problems. He had surgery on Aug. 3, 2009.