Manchester United manager Louis van Gaal admits he should have acted quicker in bringing off Michael Carrick.

The star midfielder brought himself from the Old Trafford pitch late in United's 4-2 derby win over Manchester City on Sunday after van Gaal had used all three substitutes.

It forced the Red Devils to finish the match with ten men but luckily had no ill-effect on the result as United extended their winning streak to six matches in the Premier League.

"It was a little bit stupid of me (not to substitute Carrick)," van Gaal said.

"He was wise to step off but we have to wait and see tomorrow. He had a stiff calf."