Nick Kalikas of MMAOddsbreaker.com has opened St-Pierre a minus-170 betting favorite to defeat Bisping later this year. Bisping is listed at plus-140.
UFC president Dana White announced on Wednesday that St-Pierre (25-2), who hasn't fought since 2013, will challenge Bisping (30-7) for the UFC's 185-pound championship later this year, although an official date and venue has not yet been announced.
"There are a lot of questions surrounding both fighters, especially with St-Pierre being out since 2013 and moving up to middleweight for the first time," Kalikas said. "I personally think St-Pierre was a declining fighter before relinquishing the welterweight title, based on his last few performances.
"Bisping is coming off minor knee surgery and three of his last four fights have been wars. Stylistically, this matchup should favor St-Pierre, despite Bisping have solid takedown defense and a natural size advantage.
"Bettors are extremely excited for St-Pierre's return and Bisping is clearly a villain at this stage of his career, with critics accusing him of avoiding top contenders in favor of money fights. The opening line takes all of these factors into account."
Bisping, 38, won the middleweight title in an upset knockout victory over Luke Rockhold in June. St-Pierre is the longest-reigning welterweight champion in UFC history, having defended the title nine consecutive times between 2008 and 2013.