Mills wasn't doing well with the Jays (yeah I know, an understatement). He gave up 6 runs in 3 innings in each of his last two starts. Only 1 of his 4 starts went well, his first one against Texas when he went 7 innings allowing just 2 runs. I would have rather they give him more time, soft tossing lefties tend to need time to figure how their stuff works in the majors. But there hasn't been any progress in his 4 starts.
Perez has been impressive in long relief for us. In 29 games he has a 3.71 ERA, 46 hits, 15 walks and 37 strikeouts in 43.2 innings. He's earned a shot at the rotation.
Carreno has a 3.41 ERA and a 7-9 record in 23 starts for the Fisher Cats. He has 152 strikeouts in 134.2 innings. He'll likely get the long reliever job.