The goalkeeper is facing a lengthy spell on the sidelines after his substitute appearance for the national side was brought to an abrupt halt on Tuesday
The 31-year-old replaced Iker Casillas at half-time on Tuesday but pulled up with the injury just 25 minutes later, with Pepe Reina taking his place in goal.
Spain's medical team feared he would be out of action for four weeks after completing an initial examination of the keeper, but a scan on Wednesday has revealed his absence is likely to be significantly longer.
A statement on Barcelona's website said: "The tests conducted today confirm that the player has a myotendinous rupture near the medial gastrocnemius of the right leg. Recovery is around six weeks."
Valdes has made 19 appearances for the Blaugrana so far this season, with his injury certain to ramp up the club's search for his permanent successor, as he prepares to leave Camp Nou when his contract expires at the end of the season.
He will miss Champions League clashes with Ajax and Celtic during his lay-off, as well as Liga games against Athletic Bilbao and Villarreal.