Mejia, who won't be available for a few days after throwing 42 pitches, was optioned back to Rochester after the game, with a corresponding move for another pitcher coming on Sunday.
"It was time for us to try to absorb some innings, and he was our best option," manager Paul Molitor said. "I'm grateful we had him here. It was good to see him in person as opposed to just reports. You could get a feel for what he could do, throwing it over with his mix of four pitches."
Mejia had posted a 4.66 ERA with 20 strikeouts and three walks in 19 1/3 innings in three starts with Triple-A Rochester since the deal. He was due to start for the Red Wings on Saturday but helped give the Twins some much-needed innings with relievers with Ryan Pressly, Pat Dean and Michael Tonkin unavailable.
"I felt great," Mejia said. "I thought this was a great experience for me. I worked hard for this experience. I was anxious about it, but it felt great."
Mejia, 23, is 8-5 with a 3.10 ERA with 121 strikeouts and 30 walks in 125 innings between Double-A and Triple-A with the Giants and Twins this season. Molitor said they're looking to cap him at about 150 innings after he threw 100 2/3 last year.
Mejia, a 6-foot-3, 220-pound lefty, patterns himself after CC Sabathia, given their similar statures. Mejia has a four-pitch mix, with a fastball that ranges from 90 mph to 95 mph to go along with his slider, curveball and changeup.
Vargas, meanwhile, is the victim of a roster crunch, as he's seen his playing time diminish with Joe Mauer at first, Trevor Plouffe at third and Miguel Sano at designated hitter. He put up strong overall numbers since being called up in early July, hitting .259/.379/.576 with six homers, nine doubles and 13 RBIs in 25 games. But he's scuffled in August, going 2-for-25. Even so, he is expected to be back as a September callup.
"It was tough," Molitor said. "I brought him in and complimented him on the areas of improvement he's shown. But here, with our combination of players and fit, his at-bats have been minimal. I know he's found it difficult to be sharp when [he's] used to playing every day. So you try to look at the positive; he'll get consistent at-bats over the next 10 or 12 days."
• The Twins signed reliever Edward Mujica to a Minor League deal and assigned him to Triple-A Rochester. Mujica, a 10-year Major League veteran, has a 4.76 ERA in 51 innings at Triple-A this year with the Phillies and Royals.
• The Twins traded Triple-A left-handed reliever Sean Burnett to the Nationals for cash considerations or a player to be named. Burnett had a 2.76 ERA in 29 appearances with the Red Wings.