Injured American Ben Spies abandoned an attempted return with his Pramac Ducati MotoGP team at the Italian Grand Prix on Saturday after being in too much pain to practice.
Spies injured his shoulder in Malaysia last October, missing the last two races of 2012 and initial pre-season testing. He reported muscle pain at last month's race in Texas and was absent for the Spanish and French Grands Prix.
The Tennesee-born former world superbike champion took part in Friday practice at Mugello but the team said the problems had not been resolved.
"Unfortunately yesterday's free practice sessions highlighted the muscular problems still afflicting Ben Spies," they said in a statement.
"The intense rehabilitation activities which the Texan has undergone in the last four weeks have brought about a clear improvement but still not sufficient for Spies to demonstrate his true potential on the track."
The team said the rider would have further medical checks next week.
Spies was replaced on the non-works bike by Ducati's official test rider Michele Pirro in France but that option is not available at Mugello because the Italian is already entered as a wild card.