Poulter, Westwood and Donald, who are all based in the U.S., will have to rely on a wildcard selection at the end of this week's final counting event, the Italian Open in Turin.
The Deutsche Bank Championship in Massachusetts, which ends on Monday, does not count in terms of qualifying points.
The nine automatic places in McGinley's side are occupied by world number one Rory McIlroy, third-ranked Henrik Stenson, U.S. Open champion Martin Kaymer, Victor Dubuisson, Jamie Donaldson, Justin Rose, Sergio Garcia, Graeme McDowell and Thomas Bjorn.
Poulter, the fierce, bulging-eyed competitor who galvanised the remarkable 'Miracle in Medinah' comeback victory two years ago, has had an injury-plagued season but a European team without him seems almost unthinkable.
Former world number ones Westwood, who has played in the last eight Ryder Cups, and Donald, who has featured in four of the last five editions, will also have strong claims when McGinley announces his picks on Sept. 2 despite having been some way short of their best this year.
Paul Casey, Bernhard Langer and Miguel Angel Jimenez may feel they have an outside chance of selection while Stephen Gallacher is on the fringes of an automatic place.
Gallacher is looking to provide hosts Scotland with a representative in the Sept. 26-28 matches at Gleneagles and a victory or a runners-up finish in Italy would book his ticket.
The 39-year-old was gunning for the title going into the final round of last week's Czech Masters, only to end up in a share of seventh place following a closing one-over 73.
"The week went pretty well," Gallacher told reporters on Sunday. "I was a bit disappointed with today, I didn't really play that well but top-10 if you're not firing is quite good.
"I'm heading home for a couple of days now and then I'm off to Italy," added the nephew of former Ryder Cup skipper Bernard.
DONALDSON DELIGHT
Gallacher's loss was Donaldson's gain as the Welshman clinched a Ryder Cup debut next month by winning the Czech Masters by two shots.
"It's been an amazing week," said the 38-year-old. "I didn't decide to come here until late.
"I needed to play well this week or next week or both to guarantee a place in the Ryder Cup team. It's so difficult to get in the team...I played really well and it all came together."
The nine automatic American qualifiers have already been decided.
They are: U.S. Masters champion Bubba Watson, Rickie Fowler, Jim Furyk, Jimmy Walker, Matt Kuchar, Jordan Spieth, Phil Mickelson, Patrick Reed and Zach Johnson.
Visiting captain Tom Watson will also name his three wildcards on Sept. 2 but must do without former world number one Tiger Woods who has ruled himself out.
The 14-times major champion was sidelined for three months following back surgery in March and said his form and fitness meant he would have been way short of his best at Gleneagles.
(This story refiles to make clear Deutsche Bank is not a counting event)
(Editing by Justin Palmer)
