Rossi will continue his MotoGP career into 2021 after signing a new deal with Yamaha to join Petronas SRT with full factory support.
The nine-time grand prix motorcycle world champion scored a third-place finish in July’s Andalusian Grand Prix and three more top five finishes since – though registered three-successive DNFs from the Emilia Romagna to the Catalan GP.
Speaking to Motorsport.com, Stoner notes Rossi “seems to be struggling to win” now as he commented on the Italian’s decision to remain in MotoGP next year.
“I think it all depends on Valentino; if he still enjoys the sport and maintains some speed, it’s great,” Stoner said. “It is really difficult to be in someone else’s mind.
“For me, when I was competing against Valentino, he was a winner, a killer. He did everything possible to win races.
“Now, however, a top five or a podium is like a victory for him and this is sad for me. He always did everything possible to win the races and today he seems to be struggling to win.”
Press conference: Valentino Rossi, Fiat Yamaha Team and Casey Stoner, Ducati Marlboro Team