Reo-Coker claimed he had been "hung out to dry" at West Ham, and is certain to be hugely motivated to do well against Alan Curbishley's side.
Mullins said: "Playing against him is a challenge I am looking forward to. He is probably the best midfield partner I have had, but I know his strengths, and hopefully I'll be able to combat him.
Reo-Coker was determined to leave London to put his career back on track, but Mullins revealed he had tried to persuade the former England Under-21 international to remain.
He continued: "We're trying to become a club that is pushing for Europe season after season, and I think the signings we've made prove that.
"This is a club that is going places and I feel it's the club to be at, but Nigel wanted to get out of London, and his mind was made up.
"Nigel was really impressed with Villa boss Martin O'Neill. He sold the club to him, and they have a good set-up there with some young English players.
"Everyone wants to play for the big clubs. You hear rumours about teams coming to watch, and you get distracted by what might be.
"Nigel gave everything he had for West Ham, but if there's talk of big clubs sniffing around, then you will be thinking about it."