Kolo Toure picked up a groin injury and likely to miss the United game.
Midfielder Joe Allen also suffered a hamstring problem, while loan star Aly Cissokho injured his ankle.
Gerrard said: “It was exactly what we didn’t want really. A lot of players that are going to be starting on Sunday have played too much football because I think the plan was to get the game won, done and dusted, and try to rest a few for Sunday.
“There is no doubt about it that extra-time – playing that amount of minutes – is a bit of a blow to us. It got too easy.
"I think when you’ve got a lot of inexperience in the team you take your foot off the gas and get punished. That’s what happened.”
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