The original plan was to have Lee get an extra day of rest because of his ailing back, but the left-hander was scratched altogether from his scheduled start on Tuesday.
"We're going to err on the side of caution," general manager Jon Daniels said.
So, instead it will be Hunter, who has won his last three starts. Hunter's latest win came on Wednesday in Kansas City, as he limited the Royals to a pair of runs and seven hits in 5 2/3 innings to improve to 12-2, while lowering his earned run average to 3.64.
Hunter has faced the Blue Jays twice and is 1-0 against them with a 7.50 ERA.
Lee isn't the only Ranger who is dealing with an injury concern, as AL MVP candidate Josh Hamilton will likely sit out this week with a ribcage injury he suffered while crashing into a wall on Saturday. Hamilton watched from the dugout on Sunday when the Rangers were swept in a three-game set by the Minnesota Twins, falling 6-5 in the finale at Target Field.
Matt Treanor drove in two while Julio Borbon, Cristian Guzman and Vladimir Guerrero added RBI hits in the ninth for the Rangers, who slid to their sixth loss in eight outings. However, they still lead the AL West by eight games over the Oakland Athletics.
C.J. Wilson (14-6) was tagged in the loss for seven hits and six runs, striking out five but walking four over 5 1/3 frames.
Toronto, meanwhile, avoided a sweep at the hands of the New York Yankees on Sunday, as Vernon Wells and Aaron Hill both knocked in three runs to help the Blue Jays to 7-3 win.
Wells hit a two-run homer in the first inning and Hill matched him in the third, while John Buck added a solo shot off New York starting pitcher Phil Hughes.
"It's a good win for us. We did not have a great road trip (2-4 record)," said Toronto manager Cito Gaston.
Brett Cecil (12-7) allowed three runs on seven hits and walked four over 6 1/3 innings for the Blue Jays, who lead the majors with 208 home runs this season.
Heading to the hill for the Jays today will be left-hander Ricky Romero, who is 11-8 with a 3.60 ERA. Romero picked up the win on Tuesday against Tampa Bay, as he allowed five runs (four earned) and three hits in 7 1/3 innings.
Romero tossed a five-hit shutout to beat the Rangers the last time he faced them and is 1-1 in three starts against them with a 0.81 ERA.
The Blue Jays have won five of their six matchups with the Rangers this season.