I doubt the Reds will wait that long.
If Greene returns throwing 99-100 MPH, Reds will not want to wait until his talent fades. You want to get the great arm to the bigs while it’s still great.
Once Reds think Greene can regularly pitch 5 innings, he’ll be up. He may need a reduced schedule at first, but Reds aren’t going to leave that ability in the minor leagues.
My guess is he’s on the radar screen in 2022 and will be in the bigs by OD 2023 barring setbacks.