Toronto made the decision for two reasons: because of Villanueva's innings thrown, as well as wanting to see rookie Chad Jenkins make his third start of the season.
Villanueva -- who is 7-7 with a 4.16 ERA this season -- will become a free agent in the offseason. The right-hander has never logged more than 125 1/3 innings, but proved a reliable starting option when injuries decimated Toronto's rotation.
"When the season started, obviously we were in different circumstances," Villanueva said. "I had an opportunity in the middle of the season and I feel like I took advantage of it. ... Hopefully I will get a chance to start."
Jenkins posted a 4.91 ERA in 18 1/3 innings of relief work for the Blue Jays this season, but has fared slightly better as a starter. He suffered losses in both of his previous starts, but amassed a 4.15 ERA.
The Twins also used Monday to decide on a starter for Tuesday, but due to different circumstances. Long reliever Anthony Swarzak will take the mound in place of Samuel Deduno, who left his start last week with eye irritation.
Swarzak hasn't thrown since Sept. 25, when he allowed two runs on three hits in 3 1/3 innings against the Yankees. He owns a 4.91 ERA overall, but has struggled with an 8.35 mark in 18 1/3 innings as a starter this year.
"We didn't have to use him yesterday, so he's the most rested," manager Ron Gardenhire said. "So we'll run him out there and however many innings it takes, and then we'll ad-lib from there."
Twins: Willingham's season likely over
Josh Willingham missed his sixth straight game on Monday due to a sprained left shoulder. He is unlikely to play in the Twins' final games on Tuesday and Wednesday. The outfielder suffered the injury while trying for a leaping catch on Sept. 24, and is still experiencing difficulties swinging the bat because of it.
Several normally everyday Twins players got a break from the starting lineup for Monday's game. Justin Morneau, Denard Span, and Ryan Doumit all sat out the team's series opener against Toronto, with Chris Parmelee, Chris Herrmann, and Matt Carson starting instead.
Blue Jays: Farrell dismisses report of "friction"
A Boston-area reporter indicated that Toronto is willing to let manager John Farrell go to the Red Sox for compensation because of "friction" between Farrell and Blue Jays general manager Alex Anthopoulos. But Farrell called the report "disrespectful" and "irresponsible."
Edwin Encarnacion did not start Monday's game due to tightness in his trapezius muscle. Discomfort in his neck and shoulder forced him to leave Sunday's game during the seventh inning, but Farrell hopes he'll still play again this season.
The Blue Jays' win on Monday gave them a 3-2 lead in their season series with Minnesota.
Glen Perkins was one of just two American League relievers who has not blown a save since the All-Star break before doing so on Monday in Toronto. Oakland's Grant Balfour remains the only reliever with that distinction.