Carlos Sainz Jr. believes the 2019 MCL34 looks very little like the 2018 MCL33 after driving the car in Barcelona pre-season testing.
Sainz was very optimistic about the upcoming season after many seasons of disappointment for McLaren.
The Spaniard arrived from Renault and will be partnering rookie driver Lando Norris during his first season in Woking.
"I would say we are behind Ferrari," Sainz said to Mundo Deportivo after collecting data from the first week of testing. "We're already getting more serious, but we do not know where we are (yet).
"Last year all of the average grid teams were a tenth or two (apart). When you arrive at the tests and you can that there can be half a second of difference.
"And you know how to remove 10kg from the car or putting the qualification mode or putting the soft or hard wheel you gain tenths or seconds, you create a series of variables that is difficult."
Sainz also went on to say how different this season's car is compared to last season's that was driven by Stoffel Vandoorne and Fernando Alonso.
"For me, there has to be a change of mentality (at McLaren), philosophy, a reset, a self-criticism," the Spaniard continued. "There was one at McLaren last year, but that made the tests much more prepared, with the car ready for the first day and we drove 120 laps on the first day, something that only Red Bull, Mercedes and Ferrari did.
"I do not know how long it will take us to get where we want but at least we started."