Ben Spies to sit out Jerez MotoGP race, Michele Pirro stands in

The American has still not fully recovered from a heavy fall in last year's Sepang race, which left him with an AC shoulder separation, a cracked rib and bruising to the lung.

He was forced to sit out the final two races of 2012 and the early '13 pre-season tests as a result. When he was able to return, a relatively minor off at Sepang aggravated his injuries.

While Spies has competed in this year's opening two races, finishing 10th and 13th respectively, he said he had experienced severe pain during the Austin warm-up.

"We had a little bit of a problem with a nerve in my chest, for some reason," the American said following MotoGP's Austin race.

"I don't know whether it was over-compensating with my shoulder... two laps from the end of the warm-up I had a twinge in my chest and it felt like there was a knife in it.

"I don't know what happened. We went to the Clinica Mobile; they worked with me a lot and got me ready for the race.

"We need to work harder and get back to 100 per cent because we're definitely not there, physically, right now."

Pirro rode Gresini's CRT effort last year before joining Ducati as its official test rider.

The 26-year-old Italian is set to get three wildcard starts in the 2013 MotoGP season, one of which was already scheduled for Jerez.