The Blue Jays reinstated him from the 60-day disabled list before Monday's game against the Red Sox at Rogers Centre, the opener of a four-game series.
"It seems like the last couple of days I've been a little nervous," McGowan said. "It's like getting called up for the first -- time that's what it feels like."
The 29-year-old completed his rehabilitation assignment Thursday with the Double-A New Hampshire Fisher Cats and is not expected to be ready to pitch until Tuesday.
"It's been over three years -- it feels like 10," he said.
McGowan's last appearance in the Major Leagues was a start on July 8, 2008, against Baltimore., but he left the start with shoulder pain.
The righty had season-ending surgery to repair a torn labrum July 31, 2008. While rehabbing the following year, McGowan injured his left knee during routine running drills and saw his season again come to an end with surgery July, 9, 2009.
He appeared to be making progress last year, but his comeback was once again derailed, this time needing surgery to repair a torn rotator cuff.
That might have been the lowest point of his comeback, he said on Monday.
"Last year in Spring Training when things were coming around and there was the setback where I had to have the surgery again, that kind of just took the air out of me for a while," McGowan said. "I never thought about quitting. My wife was the biggest part of it. I think it took a toll on her more than it did me."
The Blue Jays re-signed McGowan to a one-year deal worth $450,000 last December.
McGowan said the mental aspect of his comeback was the most difficult.
"Physically, I knew at some point I would get back," he said. "But it was all about my mind, you know, how long it was going to take. You have those good and bad days."
McGowan is 20-22 in his career with a 4.71 ERA. His best season was in 2007, when he finished 12-10 with a 4.08 ERA.
McGowan will be used in relief for the rest of the season.
"I hope to be a starter again," he said, "but I just want to finish this season out strong and look forward to [starting] next year."