Arsenal say Germany defender Per Mertesacker faces a long spell on the sidelines after undergoing ankle surgery only four months before the European Championship.
Mertesacker injured his right ankle in Saturday's Premier League match against Sunderland.
Arsenal manager Arsene Wenger says that "unfortunately he has had surgery and we have lost him for a while. How long I don't know. He had reconstruction of his ligaments. So it will be long term."
Arsenal are fourth in the league and facing elimination from the Champions League after a 4-0 loss at AC Milan on Wednesday in the first leg of the last 16.