Moreno had a spell at Milan during his playing career.
"Atletico need to remember that they are playing against one of the best clubs in the world," Moreno told Marca.
"Milan are a very competitive team and Atletico need to bear that in mind.
"It will be a very intense game between two great sides.
"Atletico are playing very well and are having a great season. They have a very clear idea of what they want to do and despite recent results that will not change.
"The Champions League gives you a great feeling, given that I have had the chance to play for both teams.
"I hope that the best team wins."