Has everyone not be clamoring for a RW? Is that not Sancho's primary position? If you honestly think Rashford is more at risk than Greenwood then I'm not sure what to tell you.
So you don't think form and necessity play a huge factor in who would and would not start? As things stand, there is no real cover for Rashford so he gets to play when well below par without threat to his position - playing as he has been, time on the bench would be far from improbable with a player like Sancho here. Greenwood's position in the team simply being yoinked from him without deliberation or consideration of what others are offering the team when you have an actual talent pool to select from isn't realistic.
Rashford playing to his pre-injury standard, of course, Greenwood would be the one to make way, but that hasn't been the case for quite some time now.
I'm also unsure why you believe Martial remains unaffected unless he became more consistent than he has been. Directly or indirectly, Sancho would have thrown the gauntlet down to both of these players.
Even if Sancho was thrust into the team at RW, he would have no business playing non-stop - I believe in such a congested season as we're about to embark upon, rotation will be more important than ever and it doesn't matter which of the 3 starters you pick here; all of them will need downtime and proper handling if we're to have them firing at the tail-end of the season. Sancho would have facillitated that:
Martial's excess minutes would go to Greenwood.
Rashford's excess minutes would benefit Greenwood as Sancho then goes to the LW spot.
No single player from the pack should be getting worked into the ground - I don't see how Greenwood does not get enough gametime in any scenario, even as Sancho's understudy. He's guaranteed solid playing time in two positions from three.
Greenwood's minutes need to be handled correctly even as a starter,
right now, with nobody currently at the club who is a better option than him at RW, which is what I find strange in what's being said by yourself and others.
Removing all of the talk about Martial and Rashford from my post and reducing it solely to the RW spot with just Greenwood and Sancho, the amount of games we've got coming thick and fast with so little recovery time, the two of them, in tandem, would be optimal to handle all of those games: a 40/20 split solely on RW starts, before even factoring them subbing in for the other (Sancho fades around 75ish minutes and has his own stamina to work on, which is normal given his age), that's a stacked season. How would that be detrimental?
The likelihood, however, is that Greenwood would've got more minutes as an out-and-out striker here with Sancho coming in than he will now with how things have panned out.
I think he should get as many starts/appearances as his body can handle. I'm not a football manager nor a physio, so I won't throw out a random, uninformed number but I do know I wouldn't buy a £100m+ player in the same position he starts in if my goal is to maximize his talent and see him develop.
And in our squad where Ole doesn't play stand-ins because they're terrible, and we've got two kids who barely have any experience of the adult game coming to England and the PL, do you think we're aiding Greenwood or about to put him through the ringer as Ole scrambles to save his job after a calamitous start to the season?
Greenwood is being put in like a seasoned pro and the potential backfire for that should be seen as more problematic and indicative of the state we're in than him getting solid minutes across two positions as a very important component in a 4-man rotation.