Sterling will face United this Sunday.
"I need to keep that quiet!" he told the Mirror of once being United mad.
"But when you start playing for a team, you immediately start supporting them because you want to do your best for your team and you always want them to do well.
"The older boys say that Manchester United is one of the hardest places to go. The pitch is massive and in the last 10 minutes they say it gets even bigger!
"Hopefully we can go there, do our best and come out with a result even though it's going to be very, very hard, a real test."