The Saddlers go into the encounter on the back of a five-match winless run in League One and with Mullen desperate to pick up three much-needed points.
But he knows it won't be easy, telling the club's official website: "It will be a tough game, Northampton are a decent side. They'll try and make life difficult for us and we'll have to be at our best to get anything out of the game
"A fully-fit squad is a lovely headache, from a selection point of view I've got a few problems to look forward to."
Captain Stephen Hughes has fully recovered from a virus while winger Sofiene Zaaboub managed to come through 18 minutes of last weekend's 3-0 defeat to Leeds unscathed after recent groin problems.
The postponement of Tuesday's clash against Swindon due to snow has helped his recovery and he will be pushing to be involved along with a host of others, including defender Rhys Weston.
Northampton manager Stuart Gray does not have such luxuries. He will still be without four key players.
Strikers Adebayo Akinfenwa (calf) and Colin Larkin (thigh) are joined on the sidelines by midfielders Luke Guttridge and Liam Davis, who are struggling with medial knee ligaments and a stress fracture respectively.
Gray told the club's website: "The four of them are still out but the pleasing thing is that they will hopefully be available for whenever is rearranged for.
"A few weeks ago we had players like Ian Henderson and Liam Dolman not even making the bench where as now we are struggling to get 16 fit players."
The Cobblers boss is expected to hand a place in the starting line-up to Scott McGleish after he joined the club on loan from Wycombe, having had his second Cobblers debut postponed on Tuesday when snow caused the game with Colchester to be postponed.
Sporting Life