Andrea Dovizioso has broken his collarbone and faces a battle to be fit for the first MotoGP test session of 2012 in Sepang on 31 January.
The Italian rider, who will race for Monster Yamaha Tech3 in 2012, suffered the injury in a motocross accident.
Dovizioso, 25, posted a picture of himself on Twitter in a plastercast.
Dovizioso finished a career-best third in the 2011 MotoGP world championship, his third and final season at Repsol Honda.
He will ride alongside Englishman Cal Crutchlow for Tech3 next season.
Ducati rider Nicky Hayden broke his left shoulder blade and fractured two ribs at an indoor flat track facility in December and could also miss the first test session.
The first race of the season will take place at the desert circuit of Losail in Qatar on 8 April 2012.