MINNEAPOLIS — The agent for Minnesota pitcher Francisco Liriano is asking the players' union to investigate why the Twins have yet to call him up from Triple-A Rochester.

Greg Genske says his client is frustrated because he has pitched well in the minors for more than a month but remains stuck in Rochester.

Genske says the delay has had an adverse effect on Liriano's service time, which determines when he is eligible for arbitration.

Twins GM Bill Smith says he is pleased with Liriano's progress after missing last season with Tommy John surgery. But he also noted that the team's four young starting pitchers have performed well with the big club and made it difficult to create a spot for the Dominican left-hander.