Tongan captain Inoke Afeaki has been ruled out of his side's Pool D match against Wales on Sunday.
Afeaki is a doubt for the rest of the tournament
Afeaki was knocked out in a tackle on Italy's Andrea Lo Cicero in the Pacific Islanders' 36-12 defeat.
But coach Jim Love revealed the influential back-row forward had also damaged knee ligaments.
And Love intimated the player may now miss the remainder of the tournament.
"We want to stay on track with our dream of going to the quarter-finals," said Love.
"The saddest part is that we've got a hell of a lot of players with injuries."
Tonga play all four of their pool matches in the space of 14 days.
And Love raised concerns over the lack of recovery time between each match that had left him missing five players from his first-choice squad of 22.