Valentino Rossi admits he struggled to adapt on his second bike in Argentina after missing out on the opportunity to challenge for victory.
Rossi was battling eventual race winner Marc Marquez for the lead before a mandatory bike swap due to tyre concerns saw the Yamaha rider lose his pace.
The seven-time world champion was well off the times he was showing earlier in the race, eventually settling for second as Marquez cruised to victory on his Honda.
"In the end it was a shame, because in the first part I was a lot stronger, so I thought I could fight for the win," Rossi said after the race.
"For some reason on the second bike, with the second set of tyres, I don't feel good, I had a lot of problems making the same pace.
"The conditions were very bad and I was never able to get into a rhythm, and in the end [Maverick] Vinales and the two Ducatis arrive [on my tail]."
Things could have been a lot worse for Rossi had the Ducatis of Andrea Dovizioso and Andrea Iannone not collided on the last lap.
Dovizioso was in second place before an audacious overtake attempt from Iannone saw the Italian take out his team-mate, gifting Rossi second place.
Iannone was handed a three-place grid penalty for the next race in Austin, as well as one penalty point for an "overly optimistic overtake".
Rossi said he thought his hopes of a podium place were over after losing out to the Ducatis on the penultimate lap.
"With a lap and a half to go at turn five I went very deep and both me and Iannone went wide, Dovizioso went past both of us and I thought the podium was over from me," he added.
"But at the last corner the Ducati riders go out [after hitting one another], so these 20 points are like gold for the championship."
Marquez leads the world championship standings on 41 points, followed by Rossi (33) and Dani Pedrosa (27), who finished third at Rio Hondo.
Defending champion Jorge Lorenzo is 16 points behind Marquez after an early crash saw him retire.