Tongan coach Jim Love said injuries to key players hindered his side against Italy in Brisbane.
The Pacific Islanders had the Azzurri on the rack at half-time and trailed by just two points.
But the Italians regrouped in the second period, scoring two tries through Denis Mallan.
Tonga lost both impressive flanker Ipolito Fenukitau and skipper Inoke Afeaki during the match.
Afeaki was knocked out in attempting a tackle on Italian prop Andrea Lo Cicero and was taken from the field in the 56th minute. "It was devastating to lose two top players, they are pretty hard to replace," he said.
"We definitely talked about this being a must-win game.
"But we can still make it the quarter-finals if things go our way.