Former five-time 500cc world champion Mick Doohan has warned that the clock is ticking for Valentino Rossi and what would be a record-equalling eighth premier-class title.
Rossi won his seventh 500cc/MotoGP title in 2009, but his final season at Yamaha was blighted by shoulder and leg injuries and he could do nothing to prevent team-mate Jorge Lorenzo cruising to the title.
After a high-profile winter switch to Ducati, Rossi was just seventh in his first race for the Italian factory in Qatar.
The 32-year-old then recovered to fifth after crashing - and bringing down championship leader Casey Stoner - in the recent wet Jerez race.
All of which means Rossi already trails Lorenzo by 25 points in the standings, exactly one race win.
"I think it's going to be tough for [Rossi] - he's no spring chicken in motor racing terms,” Doohan told Reuters
"I think in any sport, you've only really got a decade to totally immerse yourself in the top of a sport. He's in his 12th season [of 500cc/MotoGP] so it doesn't get any easier, especially after you've had that success.
"So I'm not saying that his career's over, but it's not a downhill ride any more, I think he's at the bottom of the slope."
Prior to the Jerez incident, Ducati's 2007 world champion Stoner had enjoyed a near-perfect start to his factory Honda career - winning from pole at round one and on pole again for round two.
"[Stoner's] platform with the Honda this year is as good as he's going to get really," said Doohan, himself a former Repsol Honda rider.
"Honda lost their way a little bit, and that's probably why they lost Rossi in the first place, and now they've decided to go back racing properly.”
Doohan won five consecutive 500cc titles for Honda from 1994-1998, but was forced to retire due to injuries sustained at Jerez in 1999.
Giacomo Agostini holds the all-time record for premier-class titles, with eight.