Ben Spies will miss next month's Jerez Grand Prix after suffering chest pain in Texas, following a shoulder injury sustained last season.
missed the last two races of last season after he damaged his shoulder in a crash in Malaysia.
He said: "I had a problem with a nerve in my chest in Texas. Two laps before the end of the warm-up, I had a twinge in my chest. It was like a knife."
Spies will be replaced in the Pramac Ducati team at Jerez by Michele Pirro.
US rider Spies went on to
finish 13th in the race in Texas last weekend,
but afterwards was advised by medical staff to do three weeks of intense rehabilitation in order to alleviate the strain in his pectoral and back muscles.
Italian Pirro, a Ducati test rider who rode for Gresini Honda on their CRT bike last year, will replace Spies at Jerez.
The Jerez Grand Prix, due to be held on 5 May in Spain, is the third round of the MotoGP championship.