Asked if Mackay will start working for Rahm, Phil Mickelson said he had no idea and didn't want to speculate on Mackay's next job.
"He's not told me anything and I think that he's going to have a lot of players call him and inquire about his services," Mickelson said. "That would be what I would anticipate. But I don't know."
Mickelson, who celebrated his 47th birthday on June 16, and Mackay started working together at a U.S. Open qualifier in Memphis, Tennessee in 1992. Mickelson was hoping to have their final round together at this year's U.S. Open, but he skipped the tournament to attend his oldest daughter's high school graduation in California.
Mackay went to Erin Hills in Wisconsin to scout the course in case a weather delay would have allowed Mickelson to make his tee time, but it didn't work out. Instead, their last round was in the St. Jude Classic — in Memphis.
"We knew that final round in Memphis that that was our last round together — or most likely," Mickelson said. "We were holding out hope that it wasn't, that we had one more week, and it was an emotional day. But we both know it's time."
Mickelson joined LPGA stars Brooke Henderson, Lydia Ko and Stacy Lewis for a skills challenge at Olympia Fields on Monday ahead of this weekend's KPMG Women's PGA Championship.
Mickelson and KPMG also announced a donation by the Phil and Amy Mickelson Foundation to the company's future leaders program, which awards college scholarships to young women and brings them to Stanford University for a leadership development retreat.
Mickelson signed autographs for about a half-hour after winning the skills competition, which included a couple different chipping contests and a knockout game involving images of each player behind plates of glass.
"I was nervous because I know how good they are," he said, "and it was fun because it gave me an opportunity to be around some of the best players in the world, best athletes in the world."