Mainoo isn't too young or inexperienced to be able to play well in this tournament, if you look at how others with similar or even less experience are kicking on (Yamal and Güler for example). It's only understandable peopel are asking for him to start (or at least feature) given how your midfield has looked, but I'm afraid he wouldn't solve the underlying issues as he'd probably look as clueless as the others on the pitch, and it's simply due to (lack of) tactics for me.
The substitutions last night also show that Southgate doesn't really have a clue on how to use his players. No wonder they all stand out for their own team under the likes of Pep, Klopp, Arteta, Emery, Ancelotti, and then look worse than players of Serbia and Denmark in this tournament. Good thing is that they are almost all young enough to flourish under your next manager but even then the 2021 Euros and Qatar WC are excellent opportunities which have been thrown away because of the FA keeping Southgate in position too long.
Mainoo is the only midfielder in the camp who instinctively triangulates the centre of the pitch or enables squares to form seamlessly. That triangulation provides much easier progression through midfield as well as the easiest out ball for players who don’t - or can’t - pass through the lines with the frequency needed for that triangulation not to be as vitally important. Mainoo would make very short and simple passes viable and, as part of a bigger chain, far more progressive than they otherwise would be. A short pass to him could instantaneously open up the field given his penchant for turning on a sixpence and having a sudden vast open space in front of him. That ability forcibly impacts upon opposing midfielders who have to factor in a different framing of the pitch and the potential fallout for not being in the correct position, should Mainoo turn out and away with the ball. In turn, that alleviates pressure on teammates because the threat such a player carries immediately becomes the priority, and that more often than not leads to shelling up and trying to protect passing lanes more so than bodies. The onus is on teammates of Mainoo’s to use that time and hesitation to work for space and to exploit the concern and anticipation. Mainoo is also adept at carrying the ball a number of yards himself, which means it’s a damnable situation whether the opposition decide to stick (and hold shape), or twist (trying to close him down directly). This team is screaming out for connective tissue like Mainoo because it has neither elite ball carriers (ala FDJ) nor an incredible DLP who by himself can negate the need for the above.
Even with a lack of tactics, struggling and panicked players know to look toward that player who takes the pressure off them with their willingness to take the ball and responsibility for it on when they themselves are flapping. Mainoo is also the only player who will put his foot on the ball, make a turn or two and slow the play down, looking for the best option, which his weaving to safety buys him the time to do. England need this more than anything else to alleviate that collective exhaustion of running ceaselessly because of such dire use of the ball, so in saying this, Mainoo by himself has the potential to solve a hell of a lot of the issues within the team, at least insofar as midfield is concerned. Off the ball running for the forwards and fullbacks should also see an uptake with someone buying time to be smarter and more selective with their runs.
Even if that route isn’t preferred, then at least put Wharton in and attempt the other method, which is, as mentioned, the deeper player who can constantly pass through the lines or go long to the diagonals. But if you are to do that, it’s imperative there are players who are eager to run behind the lines at any given opportunity, which means Gordon and whoever best fits the profile on the right.
Bottom line is neither TAA nor Gallagher should be anywhere near that midfield and whether you pick Mainoo or Wharton, there is a more defined style of play with clearer objectives for all involved. To some extent, even Southgate can’t feck that up, but it does require absolute trust in whichever foil is used to do their job without interference.