The Borussia Dortmund defender had to be helped off in the second-half of Wednesday's 1-0 win over Russia in Moscow when Cristiano Ronaldo scored the winner.
Guerreiro has confirmed he has a fracture and implies he has been playing with the injury for weeks, which he aggravated before going off in the 65th minute in Moscow.
"Thank you for your messages, I have a fracture, but it happened three months ago," Guerreiro wrote on his official Instagram account.
"Fortunately, the fracture is not causing me any pain, but I can't put my foot on the ground or move.
"I'll be behind my team-mates for the rest of the competition."
It remains to be seen whether Guerreiro will be fit for the start of the season with Dortmund with the Bundesliga to kick-off on August 18.
Ronaldo's eighth-minute header gave the European champions their first Confed Cup win in Russia and a victory over New Zealand on Saturday would secure a semi-final spot.