The Citizens drew 2-2 with Paris St Germain on Tuesday evening.
The current Bayern Munich boss is set to replace Manuel Pellegrini in the summer having agreed a deal to replace the Chilean at the Etihad.
He is unlikely to be impressed by what he has seen at the end of this season, as they have fallen out of the title race and find their top four place in jeopardy.
Wednesday evening saw them taking on Paris St Germain in the Champions League, with their defence looking shaky in the 2-2 draw, and Mills believes Guardiola will believe he has several positions that need improving.
"Pep Guardiola is thinking right now 'I need a new left-back, two new centre-backs, a holding midfielder and maybe a wide player,” he told BBC Sport.
One player that is sure to be guaranteed a place in the team however is Sergio Aguero.
The Argentine forward is guaranteed a starting place in the side, having become one of the top centre forwards in world football during his time at the Etihad.
He is a player of undeniable ability, but Mills says he has one particular problem with him.
"The one problem I have with Sergio Aguero is he's too honest at times. He tries to ride every challenge. I think sometimes he should go down,” he added.