Repsol Honda's Marc Marquez says he expects Casey Stoner to be 'fast' should the Aussie return to MotoGP with Ducati in 2016 as a wildcard rider.

Stoner, who clinched the Italian manufacturer's sole premier class title in 2007, retired at the end of the 2012 season after securing his second MotoGP crown with Repsol Honda.

The 30-year-old has undertaken testing duties for HRC since hanging up his leathers but his contract expires at the end of this year and speculation has been mounting over a possible return to Ducati as a test rider, with the potential for Stoner to compete in some races next year as a wildcard.

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Responding the news on the final day of the official Valencia MotoGP test in Spain, two-time MotoGP champion Marquez said he was 'happy' that Stoner hasn't completely cut his ties with the sport.

"Interesting, interesting... of course Casey stop very soon and of course and for sure he will be a good rider if he does many, many tests," said the 22-year-old.

"I'm happy for him because looks like he stills keeps the motivation and I think he will be fast."

Asked if Stoner's input had a significant bearing on the RC213V raced by Marquez and Dani Pedrosa, the young Spaniard added: "The thing with Casey and Honda is that he didn't make many, many tests; he test - I don't know exactly - but maybe three days, stop long time, so you know, for test rider you must follow all the items and all the things, but normally when he tests the bike the thing is that he was fast."

Stoner was leading this year's Suzuka 8-Hours race in Japan but suffered a fractured shoulder blade and broken tibia after crashing when the throttle stuck open.

HRC later apologised for the malfunction and said it would cease using the throttle specification and focus on designing a new one for endurance races.