De Gea walks in.
Valencia surely ahead of either Gomez or TAA.
Bailly with VvD is fair. Not convinced any of your left back are better than Young.
Pogba and Matic stroll into the side.
Salah definitely on the right, but Mane hasn't been better than Lingard or Martial this year.
Firmino or Lukaku up to how the manager wants to play. Firmino hasn't outscored or outassisted Lukaku, and Lukaku hasn't been at his best.
Not yet it hasn't been but hard to see Goretzka turning Bayern down unless they really low ball their offer to him.
Well of course these are all opinions. Personally I wouldn't want to change the way we play so Firmino is critical in that regard, he is what makes the team tick. I don't consider Lukaku to be a key component of the United side.
You mention Lukaku hasn't been at his best but don't accredit Mane with the same accord. For his pace, still scoring goals even when not at his best and potential I'd take Mane for sure, though Lingard has been impressive this season he doesn't play in the same position or have Mane's pace. Martial is not fulfilling his potential so far.
I like Valencia and it would be fair to select him ahead of Gomez (AA is really more of a midfielder) but as Gomez is more of a budding CB I prefer him overall to strengthen the defence. On the left Robertson and Young have the best two crossing stats in the league and no doubt Young is having one of his best seasons, but Robertson is a defensive player with attacking intent whereas Young a converted winger, I'll go with the pure defender with great potential in this instance, no doubt United fans may choose both Valencia and Young though. VvD and Bailly is unquestionable.
In midfield I also can't see any argument with Keita and Pogba, two superb midfielders who should complement each other, with Matic sitting in behind assisted by Keita and his excellent defensive stats. A team to challenge City.