Why he picks Rashford at all is a better question. Same with Maguire.
Maguire is not exactly inexplicable for me. It's mainly because he is the only one who can stay fit (AWB is another one, but he seems to be out of favour atm) in an area of the pitch where you don't want to rotate much. If we also agree that Varane is our best defender, in terms of pure quality, the fact remains that he is a covering defender who was signed by the previous regime to form a formidable centre-half pairing with the more aggressive Maguire. And when you factor in every aspect of the choices we have at the back, these two remain our most reliable CB pairing, as disheartening as this sounds to some of our fans.
Despite the shambles that led to Solskjaer's downfall, and despite the trouble we have scoring goals under RR (although it's important to keep monitoring the xG), Solskajer's game plan was always more reactive. Both FBs' roles were more supportive and whenever they were surging forward, McFred were always kept in check to cover the spaces down the wide channels. That is because none of our CBs, except for Bailly and Jones, feel comfortable when they have to defend in those areas. And they are both sick notes. The creativity and the goals would come from the front-four. Now, we want to become more systematic in our approach. We want the chances to be the product of a good structure, instead of free-role "extravaganzas". It's evident that we have moved from AWB-Shaw to Dalot/Shaw-Telles, because we need creativity from the FBs out-wide and in the half-spaces. It's also quite evident that we want our CMs to be more involved and chase the second balls and get more stuck-in in the final third. This, unfortunately, leaves us more vulnerable in the wide areas defensively, as both Maguire and Varane will struggle to cope with threats on the counter there. This is the way RR wants to build the side for the next guy, so he probably acknowledges that he's giving the CBs too much to do.
Now, i think there are two ways to interpret this. Let's take yesterday's game as an example. It's obvious that Simeone read our weakness well and targeted it. Atleti had three big opportunities in the match. They scored one, the offside trap did its job in the other, and Felix was just a touch away from scoring toward the end of the game. Great chances, but still just three throughout the game. You could argue that RR's gamble didn't pay off because, although we looked to be in control of the game, our more "courageous" positioning didn't offer us anything in terms of end product. Which can be put down to the players' quality at the FBs and CMs positions and to the fact that the transition to this new style of football is far from over. Teething pains, you could say. As an example, Klopp, after taking over the scousers midseason, had a 13-9-8 record plus 40 conceded goals in 30 matches in the PL. But he kept on pushing. And Virgil wasn't his "get out of jail" card, but rather the final piece to a puzzle that had TAA/Robertson as creative juggernauts and the gradual "enrichment" of his midfield choices. I wouldn't be surprised, if the new manager prioritizes two new FBs instead of a Maguire replacement.
Rashford came in yesterday because Elanga, minus the opportunity he had, was bad (and God, i can't even imagine the Caf's reaction, had Rashford been the one to stop his tracking run at the edge of the box just to get the best seat in the house for Lodi's goal). With the Greenwood situation and with Martial demanding and getting a loan before it, any manager's hands would be tied. Rashford, most of the time under RR, comes on as a sub when we have to throw the kitchen sink in and hope for a good moment, like yesterday or vs WHU, and he starts in games where we expect, due to the opposition's formation/tactics, to enjoy some space on his side.
