Irwin is wary of City's scoring power.
"I've got a lot of great memories playing against Manchester City," he told ManUtd.com. "It's a big, big game against them.
"They are a very good side with a very good squad and are obviously in with a chance of winning the league. City had a great result on Saturday. Even though you'd expect them to beat Fulham, scoring five goals gives them a bit of confidence going into this game.
"They have got some difficult games away from home, starting with this one, but they score plenty of goals and are really good going forward. As they are such a huge threat, we will have to defend very well in order for us to win.
"There is no doubt we can score goals, even though Robin van Persie will be a miss for us, but we can score so the main thing, for me, is to defend really well, keep it tight and try to keep a clean sheet."
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