Barnes, 24, took the roster spot of Travis Hafner, who was placed on the 15-day disabled list Wednesday morning.
Barnes didn't allow a hit and struck out one in his Major League debut against the Royals on Wednesday. He walked two and hit a batter in one inning of work, but escaped a bases-loaded jam in the sixth by getting Jeff Francoeur to foul out.
"Once [Triple-A Columbus manager Mike Sarbaugh] closed the door, he had a smile on his face," Barnes said of how he found out, "so I sort of had an idea. It was a really good moment. I'm never going to forget that."
The Indians acquired Barnes from the Giants on July 27, 2009, in exchange for Ryan Garko. Barnes had been one of the team's top starting pitching prospects, but he was recently moved to a relief role. Barnes was 0-2 with a 3.81 ERA in 11 games (three starts) for Columbus this season.
"It was just something the guys thought I could contribute to the team as a reliever," Barnes said. "I was willing to do it, and I'm excited for the opportunity to have a shot now."
Barnes said it wasn't overly difficult to adjust to being a reliever. He still throws his same three pitches -- a fastball, slider and changeup -- he just works fewer innings at a time.
"It's a little different, but at the same time, it's pitching," he said. "It's the same thing. You're just trying to get outs every time you go out there."