Manchester United are said to have been left furious following Saturday's tense 2-1 victory over West Ham.
The Red Devils saw captain Wayne Rooney dismissed in the second-half at Old Trafford for a professional foul on the marauding Stewart Downing but still managed to cling on to a hard-fought victory after visiting captain Kevin Nolan was denied a late equaliser thanks to a controversial but ultimately correct offside decision made by linesman Andrew Garratt.
While Louis van Gaal generally appeared to suggest that Rooney's dismissal was probably justified, the Daily Mail reports that Manchester United were angry following a black eye sustained by left-back Luke Shaw.
Shaw - who moved to the club from Southampton for a £27 million fee back in June and is said to earn around £120,000-per-week - made his long-awaited debut against the Hammers but sustained a facial injury during an altercation with Diafra Sakho.
The source suggests that Manchester United believe Shaw was on the receiving end of a headbutt from the Senegalese striker, although they have not made any public comment as of yet and will presumably wait to see if any mention to the incident is made in the referee's report of the afternoon which should be filed later on Monday.
No television cameras appeared to pick up the clash between Shaw and Sakho and there was no subsequent mention of any foul play during any of the highlights shows who typically dissected the action in fine detail.
If the FA - providing there is no reference to any headbutt in Lee Mason's post-match comments - do spot anything amiss and fail to deem it an accident, then Sakho could potentially face retrospective action.
Like Shaw, the 24-year-old is attempting to make an impression at a new club after joining West Ham from Ligue 2 champions Metz this summer.
But whereas Shaw has only just started to prove his worth in his new surroundings following a pre-season blighted by fitness issues, Sakho - who made his international debut earlier this year - has enjoyed an instant impact at Upton Park, notching three goals in his opening four top-flight games and forging an impressive partnership with Colombian frontman Enner Valencia.
Sam Allardyce is under pressure to deliver an improved finishing position at West Ham this year, achieved with a more attractive brand of football.
While he will have been disappointed with Saturday's defeat, the early signs have been bright for the former Bolton and Newcastle boss this season.
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