Mickelson has been a stalwart for the US Ryder Cup team for over two decades now, featuring in the previous 11 competitions.
But the 48-year-old could be at risk of being left out for the first time since making his first appearance in 1995 after missing the cut at the PGA Championships.
He shot 71-73 in the first two rounds to leave him four-over-par, missing the cut by four shots.
It means Mickelson will have to rely on one of four captain’s picks, although he is one of the favourites to be selected.
No player in US history has featured in more Ryder Cups than Mickelson, who has played in a total of 45 matches.
Dustin Johnson, Brooks Koepka, Justin Thomas, Patrick Reed, Bubba Watson, Jordan Spieth, Rickie Fowler and Webb Simpson currently occupy the eight automatic qualifying spots.
Jim Furyk will announce three of his four captain’s picks following the Dell Technologies Championship, scheduled to be completed on September 3.
His final selection will come a week later after the BMW Championship, slated to be completed on September 9.
Thobjorn Olesen, Russell Knox, Eddie Peppered and Ian Poulter are all on the bubble for the European Ryder Cup team.
As things stand, Francesco Molinari, Justin Rose, Tyrell Hatton, Tommy Fleetwood, Rory McIlroy, Jon Rahm, Alex Noren and Paul Casey occupy the automatic qualifying spots.
The 2018 Ryder Cup takes place at Le Golf National in France with Europe aiming to win back the trophy after being thrashing in Hazeltine two years ago.