According to Oddschecker, the duo are 17/2 to win in Medinah, Illinois, this week in the second event of a three-legged FedEx Cup playoffs.
Koepka, the world number one, retained first place in the FedEx Cup standings despite finishing tied for 30th last week at the Northern Trust Open.
McIlroy finished tied for sixth, the second consecutive top-10 finish after he came tied for fourth at the WGC FedEx St. Jude Invitational in Memphis, Tennessee, which Koepka won.
Behind them, Rahm is third-favorite at 11/1. The Spaniard, has shown some impressive form of late, winning the Irish Open last month before finishing tied for third last week.
World number two Dustin Johnson and Justin Thomas follow at 16/1, while defending FedEx Cup champion Justin Rose is an 18/1 shot.
Rose finished tied for 10th last week, one shot ahead of Thomas and Patrick Cantlay, who is a 22/1 shot to win at Medinah this weekend, the same odds as Patrick Reed.
Reed triumphed at the Northern Trust Open last week finishing at 16-under par, one shot ahead of Mexico’s Abraham Ancer.
The win was Reed’s first since he clinched The Masters in 2018 and propelled him in second place in the FedEx Cup standings behind Koepka.
Winning the Northern Trust Open earned the 29-year-old 2,000 points. To put the figure into context, that is 200 points more than Tiger Woods, Koepka and Gary Woodland earned between the three of them for winning The Masters, PGA Championship and the U.S. Open.
Three-time major winner Jordan Spieth is also among the 70 golfers have qualified for the BMW Championship, as are world number eight Bryson DeChambeau and world number 11 Xander Schauffele.
Spieth finished tied for sixth last week, moving up from 69th to 44th in the FedEx Standings.
The Texans carded a 67 in the final round at the Northern Trust Open, his best fourth round score this year and will need another good performance to be among the top-30 players who qualify for the Tour Championship.
At 29/1, Spieth’s chances of success are the same as Schauffele’s, while DeChambeau is 40/1 shout.
The American was at the center of controversy last week when it took him over two minutes to hit a 70-yard chip, before taking a similar amount of time as he lined-up an eight-foot putt.
His actions put renewed pressure on the PGA Tour to review its pace of play policy.
Further back in the field, Tiger Woods is a 50/1 shot to win this weekend on a course that holds special memories for him.
The 15-time major winner won his first PGA Championship crown in 1999, finishing one shot ahead of Sergio Garcia at 11-under par. Seven years later, he repeated the feat when he finished five shots clear of Shaun Micheel with a memorable 18-under par performance.
Woods, however, has been beset by injuries since triumphing at The Masters in April and withdrew from the Northern Trust Open last week after carding a 4-over par 75 in the first round.
