Vinales has picked up four grand prix wins and six poles in his two years as a Yamaha rider so far, and while he outscored Rossi in 2017 he ended up falling five points shy of the Italian's points tally in a tough '18 campaign.

Three-time 500cc champion Rainey, whose titles in 1990, '91 and '92 all came on Yamaha machinery, admitted to Motorsport.com that Vinales has so far hasn't matched his expectations.

But the American believes Vinales will be up to the task of fighting for the title in 2019 should Yamaha be able to produce a competitive package.

"Quite frankly, I expected a little bit more from Maverick," said Rainey. "I thought he would be a little more consistent.

"But it shows he’s still young, he’s still learning. All of these tough years that he has been having makes you very strong, when you are ready finally. If Yamaha produces a good bike for 2019, I think Maverick will be [ready].

"He has a lot of natural speed, he’s very confident in himself when things go well. When he struggles is when the bike setting is not right and he has to force the bike to do something he is not so comfortable doing.

"This is where Rossi is good. If the bike isn’t so good, he can still make the difference up.

"This has been one of Maverick’s weak points, but what I’m thinking now is he has to do well. He has to start beating Rossi pretty much every race."

Vinales last year struggled with his starts in particular, and often found himself plummeting down the order in the early laps and struggling to recover even after his pace had picked up later on.

Rainey believes the Spaniard addressing this weakness will be key to his 2019 hopes.

"Maverick is weak in the first part in the race, and this can be the most important part of the race, the first five laps," said Rainey. "So he has to improve in this part of the event, and this happens with experience.

"We’ll see if he improves in that area and if he does he’ll be a much bigger threat for the championship."