Ben Spies will miss the third round of the 2013 MotoGP World Championship at Jerez in Spain, next weekend.

The new Pramac Ducati rider is yet to reach full fitness following major shoulder surgery at the end of 2012.

Spies looked to be steadily rebuilding his strength, but suffered a nerve problem in his chest during Sunday's home Austin round.

"Two laps before the end of the warm-up, I had a twinge in my chest. It was like a knife, so we went to the clinic and they worked on me a lot and got me ready for the race," Spies had said on Sunday.

"I tried as hard as I could during the race, but it wasn't enough. At eight laps to go, it was either come in or salvage whatever points we could."

After the race, which he finished in 13th place, the former Factory Yamaha race winner was examined by his medical staff, who advised three consecutive weeks of intense rehab in order to alleviate the strain in his pectorals and back muscles.

Spies is thus expected to return for round four at Le Mans, while his place at Jerez could be taken by Ducati test rider Michele Pirro. Pirro, who raced the Gresini CRT machine in MotoGP last season, is already scheduled to take part in the Spanish round as a Ducati wild-card.

However, were Pirro to remain as a 'wild-card' (perhaps with some Pramac branding) he would not be using up Spies' engine allocation. The exact wording of the replacement/wild-card engine rules is as follows:

"Should a rider be replaced for any reason, the replacement rider will be deemed to be the original rider for purposes of engine allocation.

"Each Wild Card entry is allowed 2 engines for their exclusive use during each event."

Therefore, if Ducati wish Pirro to race-test new developments that require a different engine than Spies' present spec, they will need to keep his official status as 'wild-card'.