Schalke suffer Farfan and Howedes blows
In another blow for Huub Stevens's team, captain Benedikt Howedes sustained a similar injury in Germany's 3-0 win over Netherlands.
Schalke said in a statement that Farfan, who has scored 22 goals in 102 Bundesliga appearances, had torn an anterior ligament in his knee and would have to rest for four weeks before starting two weeks of strengthening work.
He limped off injured during the second half of Peru's 2-0 defeat in Quito on Tuesday, although returned to the pitch to join in a melee following the final whistle.
Schalke, fifth in the Bundesliga, said the length of Howedes's absence would be known after tests on Thursday.