Lewis Hamilton conceded not using the F-duct for qualifying in Italy proved to be a mistake after he struggled to match the leading cars.
Unlike team-mate Jenson Button, the McLaren driver removed the F-duct for qualifying, and Hamilton had to settle for a disappointing fifth position.
Button qualified in second position, just over a tenth behind pole-setter Fernando Alonso, with Hamilton over half a second behind the Briton.
"I think potentially it had quite a big impact on the end result," said Hamilton of removing the F-duct. "I'm quite disappointed, I think, with fifth place.
"We chose to go with the lighter downforce level, which doesn't have the F-duct, and the car is sliding everywhere so I struggled quite a lot on each tyre and I just didn't have the downforce.
"Wrong choice, bit of a mistake, but we'll still push hard tomorrow."
He said his final run in Q3 was not ideal either.
"I think I was probably too close to the car in front. Got a little bit in the tow of Mark Webber - I don't need it in the straight speed because I'm already quite fast there.
"I just needed to have downforce in the corners, but I was already too close to the car in front so probably a mistake there, but you learn from these and we'll keep pushing tomorrow."
And the championship leader is not expecting his McLaren team to be stronger than its rivals in race trim.
"I don't think we have an advantage. Hopefully should be quite fast on the straights tomorrow, compared to the others, and so I hope this enables me to get close but the guys [with more downforce] can go faster through the corners, so it's a real balance."