Manchester United are still evaluating the full extent of the injury captain Bruno Fernandes sustained during the 2-1 defeat at Aston Villa. Manager Ruben Amorim thinks the midfielder is set for a spell on the sidelines - but will be hoping the damage is not too bad. Fernandes pulled up at the end of the first half and felt his hamstring, before continuing and coming off at the break.

Any absence would be particularly notable given the Portuguese playmaker's remarkable availability record since joining from Sporting in January 2020. From his debut in a goalless draw against Wolves to the Villa defeat, United have played 221 Premier League games - and Fernandes has featured in 212, including 208 starts. Of the nine matches Fernandes has missed, two were because he was rested ahead of key European matches, including last season's defeat at Brentford in May. Four were because of suspension - so just three have been a result of injury or illness. His longest absence was two games in a row at the end of the 2023-24 campaign, but he still recovered before the FA Cup final victory over Manchester City.

The bad news for Amorim is United have lost the past four league games Fernandes has missed and five of the past six. While we still do not know how long Fernandes will be out, here are United's next seven games:

• 26 December (20:00 GMT) v Newcastle (h)
• 30 December (20:15 GMT) v Wolves (h)
• 4 January (12:30 GMT) v Leeds (a)
• 7 January (20:15 GMT) v Burnley (a)
• 11 January (16:30 GMT) v Brighton (h) - FA Cup third round
• 17 January (12:30 GMT) v Man City (h)
• 25 January (16:30 GMT) v Arsenal (a)

So what are Amorim's options if his talisman is ruled out? Amorim has stressed all season Mainoo is in direct competition with Fernandes as they are most similar in styles. In theory, this is the chance for the 20-year-old to show his manager he has been wrong to leave him out so often. However, Amorim said Mainoo missed the defeat at Villa with a calf problem and was unlikely to be fit to face Newcastle.

Conspiracy theorists had suggested Mainoo was left out as a result of the "Free Kobbie Mainoo" T-shirt his half-brother wore during the 4-4 draw with Bournemouth. Yet this course of action would be to ignore Amorim's admission his own brother did something similar during his own career, which led to him having to apologise and distance himself from what had happened. Amorim has not said how long Mainoo will be out for.

On the assumption veteran Brazil midfielder Casemiro, 33, will return from suspension against Newcastle, there is a chance Manuel Ugarte will partner him. Yet, like Mainoo and Fernandes, Ugarte and Casemiro have tended to compete for the same position. The last time the pair started together in the league was last season in a 0-0 draw against Manchester City in April. There is also an unhappy statistic to consider United have not won any of the past nine league games in which Uruguay midfielder Ugarte has started - a record beaten at the club only by Brazilian Kleberson.

Lisandro Martinez replaced Fernandes at half-time at Villa Park. Primarily a defender, Amorim previously said he would only consider Martinez as a midfield option "in an emergency". This situation probably meets the criteria, and Martinez was arguably Amorim's most effective performer at Villa Park. Still, neither Ugarte nor Martinez have the creative qualities that would ideally fit in alongside Casemiro.

When Mount arrived from Chelsea, Erik ten Hag felt he could operate as a number eight. Mount can create and playing him alongside Casemiro could offer Joshua Zirkzee a chance to fit in as a number 10. Amorim feels Mount is better in the number 10 role but, as with Martinez, needs must in an emergency. The England man has also played at wing-back, so the United head coach clearly admires his flexibility. The other key element is Mount is in form, having scored against Crystal Palace and Wolves recently.

Fletcher made his debut at Villa Park and has the attributes Amorim needs. Like his dad Darren - the former United and Scotland midfielder - Fletcher has lots of energy. He also has a good passing range and has made big strides over the past few months. However, it would be asking a lot to put the 18-year-old England youth international into the cauldron of a Premier League game from the start.

Since Amorim's arrival, he has attempted to get United playing his specific system, rarely straying away from his philosophy. By taking a system-first approach, Amorim looks for specific qualities from his players in each position, asking them to adapt to what he wants, rather than altering his system to fit the players at his disposal.

Looking at his system and what he wants from his midfielders may help explain how he navigates the next few months. Amorim opts for a 3-2-4-1 shape that does not rely on having the majority of the ball. At the start of his tenure, Amorim looked to build-up short, but to his credit has opted to play long from the keeper more often - which suits the players he has.

This requires players able to cover distance and win second balls. His side are asked to carry out passing routines too with patterns of play often ending in United's midfielders playing through-balls over the top for attackers running in behind. The midfield duo therefore need to be able to carry out these instructions. Amorim's preferred midfield duo of Casemiro and Fernandes provides clues to what he asks from his midfielders. Casemiro's role is more positionally strict - screening the defence, breaking up play and getting up and down the pitch to close the distance between defence and attack when the ball is played long. Fernandes is given licence to contribute in attack but, like Casemiro, has the ability to run hard up and down the pitch, getting back to defend when needed.

Both players have a tendency to play direct passes into attackers running in behind whilst taking few touches in midfield - qualities that fit Amorim's tactical plan. The other midfield options must therefore fit into either Casemiro or Fernandes' roles, with Ugarte being the most likely back-up to Casemiro while Mount, Martinez and Mainoo possess some of the quality to do Fernandes' job.

Given the constraints around the Africa Cup of Nations and United's injuries, the other option is for Amorim to tweak his system. This is not something he has done yet at Old Trafford, but - when asked about a shortage of wing-backs - he hinted he could, depending "on the way we play in that particular week, if we have to change for a back four, we change for a back four". The shortage of midfielders could force his hand in the same way.

If United are to move to four at the back, with three in midfield, the obvious player to benefit would be Mainoo. The England man has struggled to displace either Casemiro or Fernandes given the specific tactical demands Amorim asks for in this system. And the United boss believes Mainoo's strengths and weaknesses are more suited to a midfield three. "The problem is that we are playing with two [midfielders] and you guys see Kobbie in a different way to what I am seeing," he said. "Maybe if you play with three midfielders, not with just two. But with three, Kobbie will have more minutes."