Because otherwise we have no wingers? Our (realistic) order of attackers is something like:
- Rashford - LW/ST
- Martial - ST
- Antony - RW
- Sancho - LW/RW
- Ronaldo - ST
- Garnacho - LW
- Elanga - LW/RW
So if no Martial, then the 3 you pick is Rashford top, Antony right, Sancho left. If those 2 are out in addition to Martial, then the ones remaining are Rashford, Ronaldo, garnacho and Elanga. Are we going to pretend like anybody in their right mind would play both Garnacho and Elanga and bench Ronaldo from that group? I think he's the worst striker in the league and I'd never do that. It was one of the 2, garnacho had a good game vs Sociedad so he was in, which meant Rashford went wide right due to Garnacho being 18 and you don't move an 18 year old out of position.
It's simply finding the mix that works from the options we have. We have 3 wingers, one who is our 2nd choice striker.