The Frenchman was named UEFA's best player in Europe for 2012-13 after helping Bayern Munich land an historic treble last season.
Ronaldo, who won the award as a Manchester United player in 2008, is third favourite at 8/1, with Real Madrid failing to win any silverware last season.
Gareth Bale is the only British player on the shortlist but has drifted out from 33/1 to 50/1 outsider following a lack of support.
Five Premier League players make the shortlist but are all considered outsiders, with last season's top scorer Robin van Persie the shortest odds at 50/1.
Sky Bet's Football Trader Chris Spicer said: "We already had Ribery as second favourite but he has shortened across the industry, presumably due to Bayern's immense success and his UEFA award.
"Despite being short prices, Ribery and Messi have attracted a significant majority of the betting, suggesting it is very much a two-horse race."