* Argentine damages hamstring in La Liga
* Third muscle injury of the campaign
* Fabregas out for a week with knee problem (Adds Fabregas injury, details)
Nov 11 (Reuters) - Barcelona's Argentina forward Lionel Messi will be out for between six and eight weeks with a hamstring injury and their Spain playmaker Cesc Fabregas has been sidelined for seven days with knee trouble, the La Liga leaders said on Monday.
Messi suffered a muscle tear in his left hamstring and Fabregas sustained ligament damage to his right knee during Barca's 4-1 La Liga win at Real Betis on Sunday, Barca said in a statement on their website (www.fcbarcelona.es).
The double blow to the Spanish champions is perhaps not as serious as it might first appear as Messi will likely return before Barca face title rivals Atletico Madrid in January and they have already qualified for the Champions League last 16.
In Brazil forward Neymar, signed from Santos in the close season, they also have a player with a similar ability to turn a game with a breathtaking piece of skill.
Fabregas, who struck twice and set up Neymar to score against Betis before being replaced late on, should be back in action when his team mates return from the international break to face Granada at the Nou Camp on Nov. 23.
Messi will miss Argentina's friendlies against Ecuador on Nov. 15 and Bosnia on Nov. 18, while Fabregas has pulled out of Spain's friendlies against Equatorial Guinea and South Africa on Nov. 16 and Nov. 19 respectively.
The former Arsenal captain has been replaced in the world and European champions' squad by Barca team mate Marc Bartra, a central defender, the Spanish soccer federation (RFEF) said.
Messi was substituted in the 19th minute in Seville on Sunday after pulling up following a sprint for possession and a tangle with his markers. It was the World Player of the Year's 16th game of the season, in which he has scored 14 goals in all competitions.
It was the third thigh injury of the campaign for a player renowned under former Barca coach Pep Guardiola for his ability to avoid injury, despite being the focal point of the club's famed tiki-taka, one-touch football.
Barca said the first stage of Messi's recovery would be in the Catalan city before he travelled to Buenos Aires.
The 26-year-old will miss the final two matches in the Champions League group stage, against Ajax Amsterdam and Celtic.
He will also be unavailable for La Liga games against Granada, Athletic Bilbao, Villarreal and Getafe and the two-legged King's Cup tie against Cartagena but could return in time for the La Liga clash at Atletico the weekend of Jan.11/12.
Barca top the standings by three points from Atletico and six from arch-rivals Real Madrid in third. (Editing by Justin Palmer and Robert Woodward)