Manchester United defender Nemanja Vidic may face investigation after his clash with Bolton Wanderers striker Johan Elmander.

Vidic flattened Elmander during the second half of Manchester United's 4-0 win at the Reebok Stadium, a result that kept the Red Devils at the top of the Premier League.

Elmander required five stitches in a head wound and Coyle was clearly not impressed by the challenge, which went completely unpunished by referee Martin Atkinson, and as a former striker felt Vidic had gone too far.

"I can only assume the referee has not seen it," said Coyle.

"Vidic has caught him with an elbow. There was blood pouring out of it.

"I am not looking for excuses but anyone who has seen it knows it was a terrible challenge."

After branding the FA's disciplinary system "dysfunctional" last week after Steven Gerrard was not punished for two incidents he felt were the same level of gravity as the one involving Rio Ferdinand against Hull in January that ultimately earned the United man a four-match ban, United boss Sir Alex Ferguson will wait to see what action, if any, is taken against the Serb.

Key will be Atkinson's observations because if the referee claims he got a clear view, as was the case with both Gerrard incidents, the FA are powerless to act.

"It was a real battle," admitted Ferguson.

"I said to the referee at half-time it was supposed to be a game of football. He handled it the best way he could.

"You need to show composure because the ball is never out of the air and we needed some fantastic defending at times.

"I don't know how many times Nemanja Vidic headed the ball clear but it must have been thousands."

The possibility of Vidic being suspended for next weekend's crunch Old Trafford encounter with Chelsea will cast a pall over United's preparations for their Champions League quarter-final with Bayern Munich on Tuesday.