Manchester United boss Sir Alex Ferguson saw his side slip to a 1-0 defeat at Arsenal and felt Michael Owen had been fouled by Gael Clichy inside the penalty area. Although Ferguson did concede Arsenal had a good case for their own penalty when captain Nemanja Vidic handled the ball in the area.
"It's too big a game not to get the decisions right," Ferguson told Sky Sports News.
"It balances out in that respect but I think it was difficult to see that one (Vidic's handball) but the one at the end, Clichy knew, he put his hands up in horror."
Ferguson added: "Obviously, it gives Chelsea a major chance now and that's what happens when you get these decisions.
"They got one at Old Trafford last season so that's the worry now.
"But in terms of ability - a game at Old Trafford, the supporters will be ready for it - and so will our players."
The Scot conceded his team had disappointed until Arsenal scored through Ramsey in the 56th minute.
"After that we dominated and I think we deserved something," he said.
"I think we didn't use the ball properly but once we lost the goal we were in their half most of the time."
Click here for the No1 coverage of all the gossip and transfer news around Manchester United.