Back-row forward Latu, who skippered Tonga at last year's World Cup, has undergone knee surgery, the Falcons announced.
Latu, 34, was hurt during Tonga's Pacific Nations Cup campaign last month.
'Nili suffered knee ligament damage playing in Tonga's last Pacific Nations Cup game, and was operated on yesterday,' Newcastle rugby director Dean Richards told the club's official website.
'His likely return to playing is five months from now.
'Obviously it is a disappointing blow to lose a player of Nili's quality for that length of time, but at the same time it is an opportunity for other guys.
'We have a lot of players coming through - Harrison Orr has joined us and made a very good first impression - but with David Tameilau not yet here from America, there is no denying it has left us a little bit short in the back-row.'
Newcastle begin their Premiership campaign at home against Sale Sharks on September 2.