Lee Westwood's flawless 63 in the final round to finish fourth on 12 under was encouraging, while Matthew Fitzpatrick birdied four of the last five holes to make the cut.
Starting the day 6 shots behind third-round leader Casey, McIlroy roared into contention with five birdies on the front nine to pull even with Casey, who never got going and made just one birdie.
The remnants of Hurricane Hermine didn't bring any rain during Monday's final round of the Deutsche Bank Championship, but a light rain probably would have been more preferable for some golfers than the strong gusts of wind.
CRANS-MONTANA, Switzerland (AP) - Sweden's Alex Noren won the European Masters, beating Australia's Scott Hend with a 20-foot birdie putt on the first hole of a playoff.
Casey made birdie and eagle over his last three holes for a 5-under 66, giving him a three-shot lead over Brian Harman going into the Labor Day finish as he tries to win on the PGA Tour for the first time in seven years. "Things can turn around really quickly in this game and they did this week".
In his case, it only took 69 holes.
Somehow, he would fight to even par by the end of the first round, despite that big blow. He led the field in birdies with 24.
A 13-times victor on the European Tour, Casey played in three Ryder Cups but was controversially left off the 2010 team despite being one of the world's top ranked players at the time.
England's Casey, 39, missed an eagle putt on the final hole at TPC Boston that would have forced a play-off. PGA champion Jimmy Walker closed with a 70 to finish third.
Jim Furyk's hopes of a Ryder Cup wildcard look over when failed to qualify in the top 70 for Thursday's BMW Championships, reports the Mirror.
A further birdie on the 12th saw the world number five move to 15-under for the tournament. Both players were in search of their first win of the 2016 season.
It was 27-year-old McIlroy's first tournament win since the Irish Open in May and first PGA title since May 2015.
Regardless, it was also a monumental week for McIlroy with the putter. He did birdie the seventh, but missed a short par putt at the 11th and dropped another shot at 14. The wind picked up and his caddie, John McLaren, stepped him and called him off.
Walker was within two of the lead when his drive bounced into the woods right of the 10th fairway and led to bogey, and Walker burned the edges of the cup with so many other birdie attempts.