The 23-year-old Brazilian forward suffered a meniscus problem after scoring twice during the Bees' 5-2 friendly victory over AFC Wimbledon on July 20.
Thiago joined London club Brentford at the start of the month, having agreed his reported £30 million ($39 million, 36 million euros) transfer from Club Brugge in February.
"The striker will now begin a period of recovery and rehabilitation and is expected to return towards the end of the year," said a statement on Brentford's website.
Thiago scored 18 goals in 34 league games as Club Brugge won the Belgian title.
He previously played for Bulgarian club Ludogorets after starting his career in his homeland with Cruzeiro.
