Jose Mourinho surprised a lot of people by handing Marouane Fellaini a starting role in the Manchester United team in opening weeks of the new season, but the oft-criticised midfielder has repaid his manager's faith with several accomplished performances.
Unfortunately, after picking up what Mourinho described as a "bad" back injury in Saturday evening's last-gasp win over Hull City, the Belgian is facing up to a spell in the sidelines.
That means his flourishing partnership with Paul Pogba could well have to be put on hold just as the Manchester derby on September 10 comes into focus. So who should Mourinho turn to in search of a replacement in the starting XI for undoubtedly his biggest test as United manager so far?
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Central midfield is actually one of the areas of the United squad which offers Mourinho the most depth - a fact made clear by just how far Bastian Schweinsteiger is away from the team.
Options available to the Portuguese manager include; Michael Carrick, Ander Herrera and Morgan Schneiderlin. He could even drop Wayne Rooney back to the role he filled in the latter stages of last season if he was feeling desperate.
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Facing the ominous challenge of going up against the high-intensity game plan Pep Guardiola has already instilled in his City side, though, there is only one option. Herrera should be given the nod.
On the surface, the Spaniard may not provide as much defensive cover as Fellaini would have on September 10, but Herrera's capabilities when United aren't in possession are often undervalued.
Last season, for instance, the 27-year-old averaged a solid 2.1 tackles-per-game in his 27 Premier League outings. The season before, he was up at 2.8 tackles-per-game.
That figure sees him rank above Carrick (1.5) and just behind Schneiderlin (2.4). Pogba himself, although given more freedom to get forward in his United career to date, has also proven he can take on added defensive responsibilities when required, as with France in Euro 2016.
Herrera's ability to get around the park would also prove invaluable as Mourinho seeks to out maneuver his old La Liga sparring partner.
The former Bilbao player has an excellent engine and, as noted in stats taken from the turn of the year, he covers an average of around 11km-per-game for the Red Devils. Only Juan Mata ranked higher in the Telegraph's findings.
With Guardiola having quickly instilled his possession heavy, high-intensity philosophy in the blue half of Manchester, Fellaini's replacement must be able to hunt down City's players for 90 minutes if United are to get a result.
Carrick, now 35, while an accomplished midfielder, simply isn't as well equipped to do that for a full game against a team of the quality City possess.
Finding his feet
Moreover, Herrera has impressed when given fleeting opportunity by Mourinho so far this season.
In United's opening game of the Premier League season - against Bournemouth in the pre-Pogba era - the Spaniard attempted more passes than anyone on the pitch and proved that he could impress in a more defense-minded role, too.
Carrick and Schneiderlin, both starters for van Gaal for much of last season, meanwhile, will still remind some of the defensive minded football served up by the Dutchman in 2015-16.
With United having embraced a more exciting brand of football under Mourinho - and enjoyed the feel good factor it has brought - reverting back to a similar tactic, even against an in-form City side, would surely be taking a step back against their rivals.
Ideally, the Red Devils would be able to deploy Fellaini and Pogba after the international break. However, if the Belgian is ruled out, Herrera is the go to option Mourinho should take to match up best against Guardiola and co.