Chacin, who is 1-1 with a 3.65 ERA in five starts at Round Rock, replaced the disabled Tim Byrdak as the lefty in the bullpen, but manager Brad Mills said he won't be limited to that role.
"Because of what he's done and having been used as a starter and been stretched out, we're able to use him against lefties, and if we can stretch him out, we can do that, as well," Mills said.
Chacin, who had not made a relief appearance in his big league career entering Tuesday's game, is 25-15 with a 4.18 ERA in 58 career starts, all coming with Toronto from 2004-07. He won 13 games in 34 starts for the Blue Jays in 2005.
"I need to get used to doing that type of job," Chacin said. "Things are pretty much the same, because you've got to throw strikes no matter what. That's what I'm going to try to do. I've got to get a feel in that role and get used to it and throw strikes and get lefties out. That's what I'm going to try to do."