Aprilia locked out Friday practice at Rio Hondo last weekend and threatened to repeat its victory performance from the 2022 Argentina GP.
Even after a tough sprint, in which Maverick Vinales was seventh after sustaining aero damage and Espargaro crashed out, both were convinced their dry pace was still good enough to fight for the win.
However, in the wet grand prix on Sunday, both struggled massively, with Vinales 12th and Espargaro slumping to 15th.
With little wet track time on the new RS-GP prior to the grand prix, the Aprilia's grip issues in the wet came out of nowhere for Espargaro.
"It's difficult to understand," Espargaro said. "At some stages of the race I thought I had a flat tyre, it was crazy.
"I couldn't open the throttle even on the straight. Even on the straight I had to change gear at 10,000rpm before the limiter because the bike didn't go forward.
"It was crazy. Really one of my worst races and a long one. It's difficult, in Sepang we did a couple of laps in the wet, [but] not everybody rode.
"So, it was difficult [to anticipate]. But it was really a nightmare."
Espargaro added that "the mechanical grip was really, really bad, but the electronics were also not working", so Aprilia needs to find out "what is the problem and what is the consequence".
Vinales was nevertheless upbeat about Aprilia's prospects despite the difficult grand prix, as he is sure the marque's "time will come".
"It's good that they can make a reflection now, understand what we can improve and go forward," Vinales added.
"Fortunately, it's just the second race, so we can improve. The speed we have in the dry, even [in the sprint] without a winglet, was the speed to win the races.
"So, that's fantastic. For me, I'm calm. Time will come, I'm completely sure and once it comes it will not leave.
"We will keep working, we keep fighting and we need to do the best with what we had."
