Marcus Rashford has become a true game-changer for Manchester United and Jose Mourinho will hope he isn't sidelined for too long

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There was a good reason Jose Mourinho looked so anxious when Marcus Rashford limped off late in Manchester United 1-0 Champions League win over Benfica.

The United boss placed a hand on the teenager's shoulder and listened intently as Rashford explained precisely what he had felt in his knee a few seconds earlier.

It was immediately apparent that Rashford needed to leave the field, as indicated by the way he removed both his shinpads, but he did at least manage to limp off with just a guiding hand from the physio.

Jose Mourinho asks Marcus Rashford about the extent of his knee injury during the Benfica win

Rashford celebrates with his team-mates after scoring United's winning goal in Lisbon

Mourinho will hope and pray the medical checks on Rashford's knee upon United's return to England will show only a minor knock and nothing that entails a lengthy spell on the sidelines.

That's because, with every passing week and match, Rashford's importance to Mourinho's Manchester United is growing.

On a night in Lisbon on which United struggled to get above second gear, it was the 19-year-old forward that provided the propulsion they desperately needed.

Romelu Lukaku was unusually quiet, Henrikh Mkhitaryan was sloppy and Ander Herrera was not much better, meaning United often looked disjointed going forward.

Not so Rashford, whose easy touch, bursts of acceleration and ability to outwit opponents kept United's momentum going.

Having Rashford on the sidelines for a long spell would cause Mourinho a real headache

Rashford is second only to Romelu Lukaku in United's top-scorers chart this season

Rashford's shot crossed the goalline despite the best efforts of goalkeeper Mile Svilar

A clip of Rashford dribbling past Benfica right-back Douglas was doing the rounds on social media, with one tweet saying he turned the Brazilian into a fidget spinner.

And then there was the winning goal, which owed much to the inexperience of 18-year-old Benfica goalkeeper Mile Svilar but also plenty to Rashford's presence of mind.

Standing over a free-kick 25 yards out but close to the touchline, Rashford spotted Svilar too far off his line and decided to try to lob him.

By the time Svilar figured out the trajectory of the ball's flight, it was too late. The keeper caught the ball behind the goalline and his optimistic efforts to suggest otherwise were foiled by the Hawkeye technology.

It was yet another example of what Rashford offers United. That was the youngster's sixth goal of the season and, when combined with the four assists he has supplied, he is second only to Lukaku in terms of goals scored at United this season.

Rashford celebrates with Romelu Lukaku after the Belgian scored against West Ham

The young England striker was on target against Leicester City in United's 2-0 win

Lukaku has scored 11 and offered up one assist, while Anthony Martial has five goals and six assists to his name.

All three have been instrumental in United's positive start to the season and their figures serve as a riposte to this week's criticism of Mourinho's defensive approach at Liverpool last weekend.

Rashford scored 11 goals last season and so is well on course to exceed that, his excellent start another example of the confidence suddenly pulsing through the United side in Mourinho's second campaign.

Mourinho has favoured Rashford ahead of Martial in more games this season than the other way round, though both have seized their opportunities when they've arisen.

The healthy competition between the two seems to have raised standards for both, most likely as Mourinho intended at the start of the season.

Rashford seems to have a knack of scoring important European goals for United. Who could forget his debut when Louis van Gaal threw him in against Midtjylland in the Europa League in 2016 and he responded with two goals?

Rashford scored two goals in United's 4-1 win over Burton Albion in the EFL Cup third round

He also found the net as United beat Basle 3-0 in their opening Champions League fixture

On United's run to Europa League success last season, he scored an extra-time winner against Anderlecht in a ding-dong quarter-final and curled home a stunning free-kick in the first leg of the semi-final with Celta Vigo.

Rashford is becoming a game-changer and that is why United fans hope this will only be a short spell on the sidelines.

He could well miss this Saturday's Premier League visit to Huddersfield Town, but United will want him back for the home games with Tottenham and Benfica at the end of the month.

With Paul Pogba, Eric Bailly, Marouane Fellaini and Michael Carrick also sidelined, and no return dates yet for Marcos Rojo and Zlatan Ibrahimovic, a growing injury list could stall United's great start to the season.

And on the evidence of the early part of the season, Rashford could well be the biggest miss of them all.