"Big shoes to fill. And I don't think I will get close to filling those shoes," Ormsby said on Wednesday.
The 45-year-old is replacing Mickelson at LIV Golf's tournament in Adelaide at the Grange Golf Club - Ormsby's home course.
"It's a little bit different to a Saturday comp around here, put it that way," he said.
Six-time major winner Mickelson remains at home in the United States because of a private family health matter.
Ormsby, a reserve on the LIV circuit, will play on Mickelson's  HyFlyers team - the fifth outfit the South Australian has represented on the breakaway circuit.
"I have been a reserve quite a bit, you do a lot of sitting around," Ormsby said.
"And at the start of doing this role, you kind of look around at who is in the physio room and who is injured and whatever else, but that just makes you go crackers.
"So you just try to be ready and do your work and let it come to you, because you don't want anyone to go down injured or whatever else.
"But you take the opportunities when you can get them and just try and do your best when you're called upon from whichever team asks you."
Ormsby, a five-time winner on the Asian Tour, was a member on the all-Australian team in LIV's debut year, 2022. The team was initially known as Punch GC, before being rebranded by captain Cam Smith as Ripper GC.
He has since been a reserve, with the Adelaide edition starting on Thursday his 15th LIV tournament.
"When you get a chance, you haven't got too much time to think about it and you're into it, you get back into compete mode" he said.
"It's a role that I have just had to deal with and try and just take in my stride."
Ormsby said having the tournament at his home course was a "really cool" bonus.
"I do know where it goes and the slopes on the greens ... you just know where the room is and how to play each pin and whatnot," he said.
"It's definitely an advantage, but you have still got to perform and hit the shots.
"I just feel proud. You come here and everyone raves about how good the golf course is and how good Adelaide is as a city.
"And to call it my home course and my home town makes you feel proud."
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