The fullback told manutd.com: "I had a great time at Sunderland but the standard in United's first team is even higher. It helps to bring the best out of you because you have to continually raise your game.
"To be honest, I quite enjoy it! You want to play against the best and become the best yourself and marking Ronaldo day in, day out, can only help you improve. Nani is another one who's tricky to play against. He's a quality player and I can't wait to see him and Ronny bombing down the wings for United this season."