The Englishman beat out crowd favorite Colin Montgomerie and others last year to win on the Centenary Course, created by Jack Nicklaus.
Casey, ranked 20th, is missing the lucrative FedExCup after a membership misunderstanding. The second of four playoff events -- the Deutsche Bank Championship -- starts Friday outside Boston.
'I had to be a member of the PGA TOUR from the beginning of the year and I wasn't eligible,' Casey said Wednesday. 'I thought maybe there was a chance of joining once the season got going and I won enough money. But it turns out that joining would just give me temporary membership and I needed to be a full member.
'So I wanted to be part of it but there was no way ... which means I'm here, which is great,' Casey said.
In 2001, Casey won his first of eight European Tour titles when the Gleneagles event was called the Scottish PGA Championship.
Montgomerie tied for fourth last year in his first tournament after he double-bogeyed the final hole at the 2006 U.S. Open, where he could have won his first major title with a par.
Casey said he was looking forward to playing against Montgomerie.
'I relish the challenge ... especially when it is in Scotland, around a golf course I love, in front of Scottish crowds,' Casey said. 'Last year they were out supporting him and I just got the better of him.'
Montgomerie, the chairman of the tournament, confirmed he was engaged to Gaynor Knowles.
'I will either do very well because I'm relaxed or very poorly because I'm relaxed,' Montgomerie said. 'I usually do well when I'm relaxed.'
The tournament will be the final event for Ryder Cup qualification next year.
Darren Clarke, who has had a series of injuries this year and whose wife Heather died of breast cancer last year, is also in the field.
'I've had a year which would be nice to forget, in terms of golf, but I just have to play,' said Clarke, a former top-10 player now ranked 143rd. 'I've got to play tournaments. I'm trying to get world ranking points and get back up the list again.'