It's a difficult balancing act - but getting it right more often than others is obviously what separates the best managers at the top level.
Obviously, when challenging for multiple honours, the next game is always a key one and yet, at some stage, some level of rotation is necessary as you can't ask the same key players to start every match.
However, without the key game changers most capable of creating and scoring goals, then even the 'easier' games on paper often become difficult to break down a packed defence. And getting hard earned points away to the likes of Liverpool and Arsenal are great results provided you win the easier games around them.
We did that either side of the Liverpool draw with tight one goal wins over Fulham and Burnley - but by picking our key players. If we'd rested some of them, would we have dropped points there and, all of a sudden, you're looking at 2 or 3 draws and us not being top? And, likewise, a draw away to Arsenal will be great, provided we win our two home games either side of it. But in order to win games and break down packed and competent PL defences, we're still very reliant on the likes of Fernandes, Rashford, Pogba, Shaw playing well. And it's definitely not a given that we can rest them and still assume we'll have enough not to drop points.
I'm assuming Rashford will miss this game, regardless of the scan results. Or at least not start it. And I'd maybe take this opportunity to bring in Telles for a start against weaker attacking options (Shaw's the better defender to cope with the likes of Liverpool, Arsenal, Southampton, Everton so we need him for those games). But they're the only key regulars I'd consider not starting. I'd definitely go in with the likes of Fernandes, Pogba, Cavani and the rest and try to make sure we get the job done.