OAKLAND, Calif. (AP) - The Minnesota Twins have sent struggling starting pitcher Nick Blackburn back to Triple-A Rochester and taken him off their 40-man roster.

The move was made before Monday's game at Oakland. The Twins will replace him on the 25-man roster on Tuesday. Blackburn was scheduled to take the mound on Wednesday against the A's.

After a decent performance over his first two major league seasons, making 33 starts each in 2008 and 2009, Blackburn has regressed since signing a four-year, $14 million contract in 2010.

He has a 7.39 ERA in 19 starts and 143 hits allowed in 98 2-3 innings. The right-hander is making $4.75 million this season and due $5.5 million in 2013.

The Twins also outrighted infielder Tsuyoshi Nishioka off their 40-man roster. He'll remain with Rochester.