Perez was a strong fifth in the morning session but slipped down to 13th in FP2, one place behind Hulkenberg, who sat out FP1 to give way for development driver Alfonso Celis.
“In the afternoon I don’t know [what happened],” the former Le Mans 24 Hours winner said. “We’re not there with the car and the balance - where we want to be - so some homework to be done.”
Asked to specify what was wrong with the car balance, Hulkenberg joked: “It’s just crap.”
With Force India currently involved in a fierce fight for fourth in the Constructors’ championship with Williams, Perez admits he was concerned to trail their rivals, but also other midfield outfits, in FP2.
“It was a bit of shock to see McLaren, Haas and Williams seem to have the upper hand on us, so hopefully we can find some time and nice tuning of the balance.
“Generally I haven’t been very happy and comfortable with the car’s balance, so there is a lot of room to improve the mechanical balance.
“That’s our target, to figure out a nicer balance through cornering – high speed to low speed, because obviously you are running very low downforce [at Monza] so your balance is very important.”