Alexis Sanchez a symbol of Wenger's new Arsenal, and shows Manchester United what they lack

In the idle days over the international week when the politically-minded Arsene Wenger wasn’t upset by Donald Trump’s election, those close to the Arsenal manager say he was worrying about Alexis Sanchez’s fitness.

The forward aggravated a hamstring issue training with Chile last week. Rather than send him back to London, though, the national team’s set-up decided to keep him around to see if he could make Tuesday night’s crunch World Cup qualifier against Uruguay. From the very moment that Wenger received the text - and that just happened to be as the US election results were coming through - it disturbed him. It didn’t deter Chile, though, or - evidently - Sanchez.

He played against Uruguay and scored two key goals as his side came back from 1-0 down to win 3-1. Now Arsenal will assess his fitness ahead of the match at Manchester United on Saturday – but they are confident he will be able to play.

If Sanchez is available then Arsenal should also be confident of a first league win at Old Trafford in just over a decade, and Wenger’s first ever competitive win over Jose Mourinho. Because Sanchez is not just central to Arsenal’s form. He is also central to a significant difference between the sides, beyond their positions in the table. Sanchez gives Arsenal the thrust and pace that United so badly lack at the moment.

United have fallen way behind Arsenal, and not just in the table: they have also fallen behind in terms of the sheer speed the teams play at. The contrast between Sanchez and Zlatan Ibrahimovic - who is suspended for this game - almost sums it up. Whereas the fleet-footed, peak-of-his-career Chilean has found a perfect fit in a growing Arsenal side's attack, the 35-year-old Ibrahimovic has looked a little lumbering in a patched-together United that aren’t yet best arranged to maximise his qualities.

Ibrahimovic has actually worked well off the ball, and there have been flashes of fine link-up play between himself and Paul Pogba, but it still isn’t quite flowing. It still doesn’t seem as if this is their optimum set-up given the players available.

Things were different in the 3-1 win over Swansea, though, and it was all the more impressive because it went against so much pre-game logic. When United published their starting XI, it seemed as if Mourinho had gone over the squad and picked the slowest possible. It didn’t work out like that. The players available linked up to pick more angles to their attack than had been thought possible, and scored an admirable array of goals. Pogba’s was of course the best of them, as he arrowed the ball into the roof of the net.

Some at United say they feel more muscular sides like this will be picked as Mourinho goes with those he has full faith in on the pitch, but there was one big caveat to the success of that match: it came against a painfully sluggish Swansea, devoid of any drive themselves. That is unlikely to be the case with Arsenal, and especially not if Sanchez is playing.

Some close to the club even say that the specific use of the Chilean this season directly indicates how Wenger has adapted and evolved. That is of course something the Arsenal manager has been criticised for supposedly refusing to do, and has been highlighted as one big reason why he has never beaten Mourinho in a competitive match, and only rarely beaten United in the last decade: his tactical stubbornness.

This season, though, it is said he has taken on board some of the lessons of Jurgen Klopp’s press, and the spread of that type of play through the league. He uses Sanchez as a similar “trigger” for pressing. Arsenal do look a much more dangerous team without the ball these days.

In these fixtures in the last decade, Arsenal have been allowed to retain possession only to ponderously do nothing with it against poised defences. Not so recently: the way Sanchez presses, and then runs at an angle, creates space for the lively new attack to charge into. They are suddenly a real menace without the ball, waiting to burst once they have it. It has often been exhilarating.

It is also, of course, the type of thing that has been stopped in miserable Novembers in years past - or by Mourinho and United. The Portuguese came up with the defensive approach to neutralise Liverpool in October, too. Now, he will have to readjust again, but without Ibrahimovic.

Could that actually play into United's hand? Could it mean the cutting edge of Marcus Rashford or Anthony Martial on the break, who play with the pace Arsenal have often been susceptible to? Is it possible one big absence on the other side actually comes at the wrong time for Wenger, or will Sanchez's return render all that irrelevant? Will Wenger's painful week - and record against United and Mourinho - be prolonged, or will Mourinho's own troubles go on?

Much will depend on Sanchez, especially if he is the only main striker playing.