The 22-year-old signed a long-term deal with the Merseyside club to seal a reunion with manager Roberto Martinez, who left Wigan at the end of last season.
But with the Premier League side also clinching a number of other high-profile players in the window - including loan deals for Gareth Barry and Romelu Lukaku - McCarthy knows that he will have to bide his time to make an impact.
"I am looking forward to trying to prove why I am here," he said.
"It is not going to be easy, we have got a lot of competition for places. I am going to give it my best shot and hopefully I can do well.
"There are a lot of big names here. If I need to bide my time, I will bide my time."
The youngster also revealed his delight at linking up with Martinez again, under whom he started every game of the Premier League season in the last campaign.
"It is great to be here with him and hopefully I can stamp my authority on the team and get as many starts under my belt as possible," he added.
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