In 11 starts this season, Gallagher was 7-2 with a 3.39 ERA in 61 innings. The right-hander struck out 54 and walked 24. For his career, he holds a 33-14 record in the minor leagues with a 2.76 ERA in 410 1/3 innings.
Earlier this year, Gallagher received his first invitation to big league camp and made four Cactus League appearances with the club, including one start.
Though Gallagher has been a starter all throughout his entire minor league career, he expects to join the major league team in a relief role.
"I relieved a couple of games during Spring Training, and I've talked to a bunch of guys that have done it," Gallagher said late Tuesday. "It's a little bit of a different mentality, but I know I can do it. With my mindset, I'll go up there, adjust really quickly and take it from there."
The Cubs are expected to place RHP Angel Guzman on the disabled list (forearm) to coincide with Gallagher's promotion. Guzman is 0-1 with a 3.56 ERA in 12 appearances with Chicago, including three starts.
Though Gallagher admitted the news took him by surprise, Tennessee manager Pat Listach seemed none too shocked.
"I'm happy for him. He's won everywhere he's been: Midwest League, Florida State League, and he was 7-2 here," said Listach. "They're looking for somebody to come up there and help turns things around, and I think he's the guy who can help."
Listach touched on the effect on other players when a teammate is called up.
"Donnie (Veal) was excited. He was pumped," Listach said. "I tell guys every year, ‘Somebody from the team is going to the big leagues this year. Just wait and see. If you don't believe it, look back at last year. I had four guys that started with me in West Tenn. They all ended up in the big leagues.'
"I had a meeting (Monday) and I had no inkling that Sean was going to be going up," Listach said. "I said, ‘We're not playing very good at the big league level and if anybody needs some help, they're going to call down here, too; they're not just going to call Triple-A.' And then it happens.
"I just told them, ‘You didn't believe me? You can see it now. It's come to fruition,'" Listach said.
To coincide with the move, Paul Schappert will rejoin Tennessee from Triple-A Iowa. "He's going to start Wednesday," Listach said.
InsideTheIvy.com staff writer Maria M. Cornelius contributed to this report.