Ayala was 1-2 with a 4.18 ERA in 28 appearances this season. He gave up an eighth-inning home run to Houston's Lance Berkman on Saturday as the Twins relievers let a 3-1 lead slip away in a 6-5 defeat.
The Twins signed Ayala to a one-year, $1.3 million contract in February in hopes that he would develop into a reliable setup man for closer Joe Nathan. But Ayala allowed 38 hits in 32 1/3 innings this season.
The Twins say they will bring up right-hander Bobby Keppel from Triple-A Rochester in time for Tuesday's game at Milwaukee.