Mickelson missed his beloved Masters - where he's won three times - in April due to an unspecified personal health issue within his family.
It appears the issue is unresolved as the six-time major winner will not be in the field at the 2026 PGA Championship on 14 May at Aronimink Golf Club.
While Mickelson hasn't publicly commented on his absence, LIV Golf CEO Scott O'Neil said he spoke with Mickelson on Monday.
"I just want to be really sensitive," O'Neil told ESPN.
"We all have things going on in our lives that are personal, and this is. I have a lot of love and respect for him as a human being, and certainly have respect for him for what he's accomplished in his career. His presence is really helpful. I hope he comes back soon.
"I miss his presence at events. He carries a ton of weight in every room he walks. That's not only because he's a six-time major champion, but he's got gravitas. It comes with something that's way beyond winning. He's one of the guys helping me, of course."
As a result of Mickelson's withdrawal, Max Homa will now be in the updated 154-player field when the major tees off next week. Homa, a former world number five, has suffered a dramatic loss of form until recently. The American slipped down to 150 in the rankings before a superb T9 finish at the Masters last month.
In addition, the PGA of America granted a special exemption to two-time major winner Dustin Johnson.
"Very excited," quipped Johnson.
"I want to play in all the majors. I feel like my game is still good enough to compete with everyone and still can win. The PGA was nice enough to give me an invitation. So yeah, I'm very, very happy and thankful for that."