Surgeons said they were ‘surprised’ at the depth of the cut, which looks set to keep the England star out for a month.
Rooney remained in hospital overnight as medical staff assessed the extent of the injury he sustained when Hugo Rodallega landed on him in stoppage time.
Immediately after the game, Sir Alex Ferguson said it was ‘a very bad one’.
It seems inevitable he will miss England’s opening World Cup qualifiers against Moldova and Ukraine next month.
Roy Hodgson is due to name his squad on Thursday, at which point it will be surely too early for any risk to be taken with Rooney’s recovery.
The one reason for optimism is that when Nani needed stitches in a nasty wound, admittedly below the knee, inflicted by Jamie Carragher at Liverpool in March 2011, Ferguson immediately put the recovery timescale at three weeks, yet the Portugal winger was back in action nine days later.
With the start of another Champions League campaign and a trip to Liverpool looming straight after the international break, any reduction in that four-week timescale would be a bonus for Ferguson.