Rooney could be charged with violent conduct for his apparent elbow to the head of McCarthy in his side's Premier League win on Saturday, a crime that could carry a three-game suspension.
The Football Association will release the contents of referee Mark Clattenburg's report on Monday morning, just a day before United are set to take on Chelsea at Stamford Bridge.
Rooney lifted his arm into the side of McCarthy's head early in United's 4-0 victory and Clattenburg gave a free kick at the time. The case is a test of the FA's disciplinary system, with retrospective 'trial by video' only to be used for incidents which escape the attention of the referee.
Even if he is found guilty, Rooney could be free to face Chelsea if he decides to appeal against any charge. That would risk the possibility of an extra match suspension for a frivolous appeal but given Clattenburg's decision not to book Rooney on the spot for an incident he had to be at least partly aware of, United could forseeably go down that route.
At the very least, it brings uncertainty United could do without ahead of one of their most important matches of the season. Chelsea might be 15 points behind the Premier League leaders but, as Edwin van der Sar acknowledges, they will be desperate to win, not just to maintain an outside chance of retaining their crown, but to claim a significant scalp in an otherwise difficult season.
"They will be looking for our scalp,'' said United goalkeeper Van der Sar. "We would be the same in their situation. We know they will be eager to beat us, that is obvious.''
United know victory would end Chelsea's title hopes, whilst striking a significant psychological blow on Arsenal in the wake of their shock Carling Cup defeat. Not that Van der Sar is taking anything for granted in this strangest of seasons. After all, it is not that long ago that Chelsea were five points clear and apparently ready to stroll to glory once more.
"The fact we overturned their lead does also serve as a reminder that it can be done to us,'' said Van der Sar. "It is better to be on top but four points is not a lot with 11 games to play. There are still 33 points to be had, so you can't get too far ahead of yourself. Chelsea are a little behind points wise but you know who your closest competitor is and you have to stay in front of them.''