The thing is that with Ronaldo we can't play the 343 without consequences. This will leave us with the same problems as the 4231 where he is playing as kind of a lone striker. He needs a strike partner.
So when Ronaldo plays we will most likely play best in a 352.
His best strike partner at United is obviously Cavani. The one player I wouldn't have minded seeing a bit is Greenwood yesterday alongside Ronaldo after our 2-0 lead to see if arguably our 3rd best striker (after Ronaldo and Cavani) could start to work in striker partnership's especially since he is left footed.
Either way, I think Ole went for Rashford primarily (don't even know if Greenwood was kn the bench) because Rashford's the most winger like player out of the 4 and needs to get in to the rhythm of playing as a centre forward again if we are sticking with a 352.
Then it comes to Sancho - I've heard he has played some games centrally for Dortmund so it won't be shocking; it's just a bit unfair on him to maybe come and play that role outside an FA Cup match considering he has hardly played a LW or RW match for us either. It's more something I'm wanting to see because of his dribbling, his ability to go on both flanks and ability to play through balls.
I think we have to give Ronaldo and Cavani the 352 partnership and slowly build that team up (aka see if Pogba is able to fit in midfield or if he still leads us to feel like we need extra defensive balancing).
Let's the youngest players sit out a while and come on as subs - Greenwood and Ronaldo needs to try and get in to some partnership because those are the most natural strikers we have if anything happens to Cavani.
In the FA CUP we should play a flat 343
Rashford- Greenwood- Sancho
To give the winger chance to the players in their normal position in my opinion and see how that works with a back 3 as an alternative.
Depending on who is available, who gets injured - we need to fit the right tactics to fit the certain players we play.