Keane was superb in the victory against Newcastle United in the previous round of the competition.
"You play games with more pressure when you get to a higher level," he told manutd.com. "Every game you play, you've got more experience of playing for points and playing because it means something rather than just friendlies. When you get called into the first team, you're used to being desperate to win and getting the three points [or getting through].
"I was so tired [after the Newcastle tie]. I've never been that tired after a game. Mentally as well as physically because of the concentration, how the day went building up to it and then the 90 minutes was tough but we did well.
"Obviously it was harder physically being on the big Old Trafford pitch but mentally playing against more experienced players means you have to be concentrating the whole time and not taking any breaks. It does take it out of you but, as long as you concentrate for 90 minutes, you're fine.
"Shola Ameobi has played in the [Barclays] Premier League for years so you know if you're playing against him, you're probably good enough to play in the Premier League. I see it as a challenge to yourself and the whole back four coped reasonably well."