Northern Ireland's Jeremy McWilliams suffered multiple injuries in a crash during the final MotoGP test session of the year in Jerez, Spain.
McWilliams' last full season in MotoGP was 2004
The 42-year-old was taken to hospital after fracturing the femur in his left leg and his collarbone.
McWilliams, who was riding a new Ilmor 800cc bike, also suffered injuries to the fingers of his left hand.
The official MotoGP website said he is expected to undergo surgery next week. The new MotoGP season begins in March.
Repsol Honda's Dani Pedrosa clocked the fastest overall time on Thursday.
The Spaniard was the only rider to clock under one minute 40 seconds for a lap of the 4.423km track, with a best time of 1:39.910 secs.
Former champion Valentino Rossi was second fastest in a time of 1:40.123, with Marco Melandri in third.
Nicky Hayden, the 2006 world champion, missed the test for shoulder surgery.
No testing is allowed now until the end of January, with the 2007 season kicking off in Qatar two months later.