MILAN, Sept 13 (Reuters) - Ronaldo will miss AC Milan's trip to Siena on Saturday because of a nagging thigh injury that doctors are struggling to treat.
The Brazilian has not played so far this season and recently flew to Antwerp and Brazil to seek second opinions with Milan admitting they had misjudged the injury.
Coach Carlo Ancelotti had said Ronaldo would definitely return after the international break but he will again be without the striker against Siena, who saved themselves from relegation with five minutes to go last season.
Juventus, who along with AS Roma are the only side with two wins from two this term, host Udinese on Sunday without injured defender Jean Alain Boumsong and the suspended Jonathan Zebina.
French defender Zebina has received a four-match ban for being sent off and trying to slap a steward's face in his side's 3-2 win at Cagliari two weeks' ago.
Udinese, battered 5-0 at home by promoted Napoli last time out, will hope Antonio Di Natale can continue where he left off on Wednesday when he scored both goals in Italy's 2-1 victory over Ukraine in Euro 2008 qualifying.
Roma visit Reggina on the toe of Italy with Azzurri players Christian Panucci, Simone Perrotta and Alberto Aquilani set to feature after coming through the Kiev win relatively unscathed.
Patrick Vieira captained France in Saturday's 0-0 draw with Italy and the 1-0 home defeat by Scotland on Wednesday despite Inter Milan saying he was not fit following hamstring trouble.
The midfielder criticised Inter and it remains to be seen if he will reclaim his place for Sunday's home game with Catania.
'I have never chosen between club and country. If I'm alright I play for one and for the other,' the 31-year-old midfielder told a news conference on Tuesday.
'If Inter prefer to speak via newspapers instead of telling me what they think, it is their problem. I don't usually talk about this sort of thing in the newspapers. I'm disappointed.'
Champions Inter are definitely without centre back Marco Materazzi, who continues his recovery from a thigh injury.
Lazio host Empoli on Saturday with new keeper Fernando Muslera pushing for start after 43-year-old Marco Ballotta kept his place in the 0-0 draw with Sampdoria on Sept. 2.
Bottom side Livorno, who have conceded nine goals in their two defeats so far, visit promoted Genoa on Sunday.