Andrew Cashner, the Cubs' top young pitcher, suffered a setback ­Monday in his efforts to return from a shoulder injury when he experienced "tightness'' in the shoulder before a scheduled extended-spring-training start. Cashner, who went on the disabled list April 8 after leaving his season debut with soreness, was expected to have another MRI exam in the next day or so. The initial MRI exam revealed no structural damage in the shoulder, and team officials say the hard-throwing right-hander had sailed through rehab mile markers without a glitch until Monday. He was expected to start a minor-league rehab ­assignment next weekend until the start was scrubbed. Cashner, a closer in college who became an effective late-inning reliever as a rookie last season, spent this spring building strength and stamina for a job as the fifth starter. For now, the Cubs say they have no plans to change that role when Cashner returns, even in light of this setback. "Cashner's a starter as far as I'm concerned, and we'll make adjustments [in the rotation] when the time comes [for him to return],'' manager Mike Quade said. "And I don't know what the hell would change that, but ­nothing that's happened to this point, that's for sure.'' The Cubs optioned struggling outfielder Tyler Colvin to Class AAA Iowa after the loss Monday to the Cincinnati Reds. The Cubs didn't immediately announce a corresponding move.