Nzonzi has struggled to replicate the form which won him the club's player of the year award last season but Allardyce is confident the Frenchman will bounce back.
"Perhaps he got a little bit too much praise last year, but he has to just get his feet back on the ground and get back down to basics, reproducing the sort of game that we didn't expect last season for as long as we got it," Allardyce said.
"He overachieved last year and that brought him many plaudits which he deserved, but this year he has found it a little difficult to get back there.
"It is all about your mental strength. He probably thought it was going to be a little easier this year, but this league is never easy and if you take it a little bit easier, unfortunately, you get found out.
"He needs to just kick back into the mode he was in last season.
"We were all expecting him to improve - he has had a little dip at the moment as many youngsters do, but I'm sure he'll be back very soon."