"We are making away games harder for ourselves by not taking our chances.
"We could have won by six at Reading and ended up drawing, which was a slip, and we should have won by a lot more at Sheffield United and Middlesbrough.
"Overall, the players have been doing fantastically and I have to trust them, but it is an area where we can improve.
"I would take that right now.
"There is no doubt Sunday will be a harder game for us now," he said.
"The West Ham players will have an increased energy and desire because they will want to convince and impress their new manager.
"But maybe that will be a good thing for us because we will have no illusions about the task that awaits us."
New West Ham boss Alan Curbishley should have Anton Ferdinand available again after the defender's recovery from an ankle injury.
But Danny Gabbidon (hamstring) is still out and striker Dean Ashton (ankle) will not be ready to return until at least next month.
Patrice Evra remains Manchester United's only injury absentee.
That means Sir Alex Ferguson could name an unchanged side from the one that beat Manchester City 3-1.
West Ham (from): Green, Carroll, Konchesky, McCartney, Ferdinand, Collins, Spector, Bowyer, Reo-Coker, Benayoun, Etherington, Tevez, Harewood, Mascherano, Zamora, Sheringham, Pantsil, Mears, Cole, Noble, Dailly
Man Utd (from): Van der Sar, Kuszczak, Neville, Brown, Ferdinand, Vidic, Silvestre, Heinze, Ronaldo, Fletcher, O'Shea, Scholes, Carrick, Giggs, Park, Richardson, Rooney, Smith, Solskjaer, Saha.
The Premiership leaders will be looking to consolidate their five-point advantage over Chelsea, and Sir Alex Ferguson could name an unchanged side.
United have an impressive away record this term, failing to win on just one occasion - at Reading - although with Alan Curbishley taking charge of the Hammers for the first time another victory will not be easy.
Kick-off at Upton Park is 4pm.
United (from): Van der Sar, Kuszczak, Neville, Brown, Ferdinand, Vidic, Silvestre, Heinze, Ronaldo, Fletcher, O'Shea, Scholes, Carrick, Giggs, Park, Richardson, Rooney, Smith, Solskjaer, Saha.
The Beeb say it should be 9°C and sunny. We are expecting to be dark at or around 4:05pm.