Pramac Ducati rider Danilo Petrucci said he has "doubts" about Ducati's 2018 MotoGP bike, with the factory riders remaining split over whether the GP18 is better than the GP17.
After Andrea Dovizioso won the Qatar season opener, no GP18 finished inside the top four in Argentina and Austin, with Petrucci and Jorge Lorenzo even failing to make it to the top 10.
Ahead of the Jerez weekend, Petrucci pointed out that GP17 riders Jack Miller and Tito Rabat have looked no slower than the current-spec bikes in the recent races.
"Now in the last two races for sure we have this doubt at the moment," said Petrucci.
"After the first test in Qatar, it was very similar. Maybe with the GP17 you have to ride two-three laps for make a good laptime - during testing with the GP18 at the first or second lap you already found the right feeling.
"Then in Argentina we start to have some problems and we found the GP17 more competitive, both Miller and Rabat have been competitive in Argentina, maybe more than me and Dovizioso.
"For sure Jack did a great weekend in Argentina but even Tito was pushing Dovi until the last corner.
"For me, Austin was a disaster race - but we saw that with GP18 we change a lot of things, we struggle to find the good set-up.
"At the moment there is not a big difference. Maybe you have an advantage with the GP17 because you already know which direction to go.
"Like every year, the new bike at the beginning of the season is to improve and develop."
Lorenzo, who has endured a nightmare start to his campaign, said the GP18 is creating more issues for him, and admitted that "the connection with the bike is not the best one".
"Last year I had a great progression in the last seven races, eight races, was coming better and better," he said.
"Unfortunately we changed the bike and this bike looks like was really good in Sepang to make one, two or three laps very very quick - in fact, I break the record of the track - but also this bike creates some more problems than the bike from last year.
"And for the moment the connection with the bike is not the best one.
"I had some, a few good moments during these three races that I could demonstrate my speed, but not really really fast, really really quick like I would like to be."
Dovizioso, on the other hand, disagreed with Lorenzo, insisting the current bike is a step forward in terms of turning.
"I don't agree. I think the difference from the bike of last year is very small.
"For my feeling, for my riding style, in the middle of the corner I think is a little bit better.
"There isn't a big change from last year, but is very, very similar than last year, and I didn't feel a negative point from the 2017.
"Also with the result I did a similar or better result than last year, so for me is quite okay."
Additional reporting by Matteo Nugnes