The Chilean started last night’s game in the same position that he favoured at Arsenal, from whence he arrived amid much fanfare just over a week ago. Out on the left with a licence to drift inside and cause havoc.
The problems were not singular, however. Tottenham’s start — scoring within l1 seconds and pegging back United with their high-energy high press — meant Sanchez barely got a chance to get into the game.
It also meant pushing Anthony Martial out from his favoured left wing to the right, where he was nowhere near as effective as he can be.
Mourinho rectified the situation at half-time, putting Martial back on the left, but by then the damage was done. Apart from an early flurry of chances that required quick thinking by Hugo Lloris, the Tottenham keeper had one of his quieter nights.
It was not all the fault of Sanchez, of course. There is only so much one man can do, and United were collectively outfought and out-thought by Spurs, who would have won by more if Harry Kane had brought his shooting boots and if Andre Marriner had been less lenient with Antonio Valencia when the United defender swept Dele Alli’s feet from under him ten yards from goal.
Sanchez barely had a chance to get going, but then neither did any of his team-mates. Romelu Lukaku had another frustrating night, either caught offside too often or comfortably shackled by his compatriot Jan Vertonghen, who had an outstanding game.
Jesse Lingard was quiet, too, and Ashley Young had a big enough job on his hands simply trying to contain a rampaging Kieran Trippier down United’s left.
It was the collective failure of Young and Sanchez to track back on Trippier that allowed the Spurs right-back to gallop into space in the 28th minute and deliver the cross that Phil Jones miscued horribly into his own net.
Paul Pogba was one United player who more than held his own, but he was curiously withdrawn in the 63rd minute to make way for Marouane Fellaini, who was then humiliatingly hauled off by Mourinho seven minutes later.
It suggested confused thinking by the manager, who was outwitted comprehensively by Mauricio Pochettino, the man that some of the power brokers at Old Trafford would like to employ.
They would also like a clutch of Tottenham’s players too, including Kane, Alli, and Eric Dier, but those young England stars look just fine where they are.
It is no exaggeration to say Spurs played United off the park and long before the end, the home supporters were giving it the big “Ole” as their team strung tidy passing moves around their opponents, who were run ragged in the final stages.
Spurs are one of those sides chasing down United for a place in the top four, and on this form they look capable of catching them. They are still five points behind United but have big players such as Tony Alderweireld and Danny Rose about to return from long-term injury, as well as the addition of exciting winger Lucas Moura, signed from Paris St Germain for £25m (€28.6m) yesterday.
So the momentum is certainly with Tottenham, while Mourinho must be wondering what he has to do to get United firing again.
And his first priority has to be working out a way to get Sanchez firing like he did for the Gunners. When Alli dumped him on his backside late in the game, and referee Marriner showed no mercy by waving play on, one had to feel for the Chilean, whose every touch was booed by the home supporters because of his Arsenal past.
His future is very much with United — but on the mammoth wages that they are paying him, they will want to see better results than this. And soon.