Koepka recently handed the Northern Irishman a lifeline with the rival league by signing the former European Ryder Cup star to his Smash GC team.
McDowell was out of contract having only penned a two-year deal with the breakaway tour.
The 44-year-old was part of Martin Kaymer's Cleeks for the first two campaigns but faced an uncertain future at the end of the 2023.
It is not clear why McDowell wasn't retained by the German but sources close to the Irishman have suggested the duo didn't really see eye-to-eye.
Koepka officially welcomed McDowell to his side on 30 November with a short video clip.
Golf fans also pointed out Matthew Wolff wasn't featured in the clip, which suggested his public feud with Koepka hasn't been resolved.
One golf fan wrote: "No welcome from Wolff?" Another added: 'Was Matt Wolff busy that day?'
Another went in hard on Koepka, writing: "Contrived s---. Absolutely hilarious. Transfer window with a closed shop. Nobody watches."
Koepka said in the clip: "G-Mac, what's up? It's Brooks. Just wanted to say congrats on signing with team Smash.
"We're excited to have you. Can't wait for this year.
"I think it's going to be an awesome year. Can't wait to have you part of the team. Let's go team Smash."
Watch the clip here:
- PGA Tour winner rips (!) into commissioner after entering LIV Golf Q-School
- LIV Golf Promotions: Full field and prize money information
- Golf reporter daggers Phil Mickelson by publishing illuminating email exchanges
- LIV Golf's Dustin Johnson sinks to remarkable career low