Alexis Sanchez finally looked the part for Manchester United against Tottenham last week, and it is not hard to explain why: he was at peak fitness for the first time this season. You could see it in the brilliant goal he scored. He guided the header perfectly into the corner, but I was just as impressed by the athleticism he showed to rise to meet the ball, and the speed he showed to be in that position.

He is only just recovering from a difficult start to the season, when he was blamed by people in Chile for their struggles in World Cup qualifying. Fans there even tried to stage a protest march against his girlfriend, who they claimed had taken his focus off football.

Blaming the girlfriend was clearly harsh, but there is no question that he did not look fully fit during the first half of the season, or after he joined United in January.

At Arsenal, he could get away with it more because Arsene Wenger did not ask much of him defensively. But no wide man will get an easy ride playing for Jose Mourinho. The list of wingers who have fallen by the wayside because they did not show the discipline Mourinho demanded is a long one and includes - at Chelsea - Arjen Robben, Joe Cole, Mohamed Salah and Juan Cuadrado, plus United's Memphis Depay. Anthony Martial looks like he may leave United this summer.

If you look at the stats, Sanchez is making more tackles under Mourinho than Wenger, and is spending a bit more time in deeper positions, helping his full back. That is non-negotiable under Mourinho.

I think that when he joined United, his fitness level wasn't high enough to thrive in Mourinho's system. Last week at Wembley, that tigerish attitude, the ability to chase down defenders and pressure them into mistakes, was back.

He has scored fewer goals per game than at Arsenal but, again, that will happen under Mourinho. Last season was his best yet for goals - he scored 30 - but that is not going to happen at United, where he is being asked to provide more assists for Romelu Lukaku. His role at Arsenal was more selfish, while at United it is selfless.

I fancy him against Arsenal's back four - and, aged 29, he has another two or three years of his best to give United.

Part of him may wonder what would have happened had he gone to Manchester City. But, despite taking until now to reach his best level, he will know leaving Arsenal was the right move. Arsenal's transition is only starting now, and it may take years to succeed; Sanchez didn't have time to wait for them.

THE TIMES, LONDON