Manchester United medical staff are carrying out extensive tests on the back injury suffered by Ander Herrera in Saturday's 2-1 win over West Ham United, but according to The Guardian are yet to determine the full extent of the problem.
Herrera complained of back pain in the second half against the Hammers and was replaced in the 74th as a precaution, although United were quietly optimistic that there was no long-term problems.
That positive outlook has given way to fears that the Spanish star could be facing a spell on the sidelines, with Louis van Gaal now anxiously waiting to find out if United's ongoing injury criss has deepened still further.
Van Gaal's early tenure plagued by injury problems
News of a potentially serious problem for Herrera is just the latest in a long line of injury problems for Van Gaal, is already without a string of potential first-team regulars including Phil Jones, Chris Smalling, Jonny Evans and Michael Carrick all still sidelined.
Throw in captain Wayne Rooney's three match suspension and Van Gaal is fast being robbed of the chance to make selection decisions purely for tactical reasons, with the presence of unheralded youngster Paddy McNair in the starting XI a clear sign of that.
Losing Herrera for any length of time would represent another significant blow to Van Gaal's plans, with the £28.4 million midfielder quickly settling to life at Old Trafford despite niggling injury problems.
The 25-year-old has scored twice in four games and definitely been a rare bright spot for the Red Devils so far this term, but has only been able to complete two of their six Premier League games early in the campaign.
United fans will be desperate for news on Herrera's fitness ahead of this weekend's Premier League clash with Everton, although at this stage it looks highly likely to miss out.
Mercifully United's absence from European competition this season does give them some breathing space to get key players fit and ready to meet the challenge of facing something of a bogey side in Everton on Sunday afternoon.