Petrucci had led a Ducati 1-2-3-4 in this same test last year, but was only sixth-fastest over the past three days - with the lead Desmosedici of Pramac rider Francesco Bagnaia in fourth.
More pertinently, neither Petrucci nor works teammate Andrea Dovizioso managed to consistently lap within the 1m59s margin during their 10-lap race run midway through the final day - with the likes of Maverick Vinales (Yamaha), Marc Marquez (Honda), Alex Rins (Suzuki) and even Aleix Espargaro (Aprilia) looking more reliably fast.
"For sure, in the laptime, I am not so fast," Petrucci said. "I am not too far from the lead but one year ago I remember we were four Ducatis [up front], I did the record lap - but it's not the thing that we are looking for.
"I think for example Maverick and Marc do completely another type of work, and I think they are the two fast riders, also Espargaro is very fast, Rins, [Fabio] Quartararo.
"There are a lot of riders able to ride more than 10 laps in [1m]59[s]. And we're struggling to keep this pace."
Asked whether he was referring to Aleix Espargaro or his KTM-riding brother Pol, Petrucci quipped: "Both, unfortunately."
The Italian thus conceded that he was "for sure not fully satisfied" with how the test had panned out.
"Still some work to do, and for sure there are many many riders faster, and I'm not happy about it, but at least we understand many things, and this is good."
Dovizioso, who was 15th-fastest but had set his best time on a harder tyre, was more positive but admitted "the feeling was not that good".
"What we can see from the test is that Maverick is really strong in the simulation," Dovizioso sumed up. "Rins, really comfortable.
"And we are not that far, so this is good. We did our best lap with the hard tyre, like [Pramac rider] Jack [Miller], so this is good. But still confidence is not that good."
Additional reporting by Oriol Puigdemont and Lewis Duncan