Ferguson (4-2) is minus-405 against Shamrock (28-15-2), who is plus-285. The lines were set by Nick Kalikas of MMAOddsBreaker.com.
Ferguson and Shamrock were originally supposed to fight in October 2008 on an EliteXC card. The bout was pulled on the day of the event due to an accidental cut suffered by Shamrock in his locker room.
"For the original meeting, Kimbo Slice was around a 5-1 betting favorite heading into the fight and had all of the public support behind him," Kalikas said. "Several years later, it's 51-year-old Ken Shamrock who is receiving early support from the betting public."
Neither heavyweight has competed in MMA since 2010. Shamrock has been inactive, while Ferguson has appeared in seven pro boxing matches, compiling a 7-0 record.
The bout headlines Bellator 138, which takes place at the Scottrade Center in St. Louis.