Rooney, who surprisingly started on the bench rather than partnering new signing Robin van Persie up front, had blood streaming from his leg after being caught by the boot of Hugo Rodallega and had to be carried off on a stretcher.
“He’s got a very bad gash. It looks as if he’ll be out for about four weeks. Obviously you’ve got to be careful with these things because of infections,” Ferguson told Sky.
England forward Rooney, now doubtful for the first Champions League group game in mid-September, had started Monday’s opening 1-0 defeat at former club Everton when former Arsenal striker Van Persie was on the bench.