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Openside flanker Lentjes has a prolapsed disc in his neck and Larsen dislocated his elbow in a warm-up game.
Otago coach Ben Herring confirmed yesterday it was unlikely they would be seen this season.
With the short duration of the Mitre 10 Cup, injuries, no matter how serious, could end the season for players.
Larsen has to rest ligaments he had damaged in his elbow while Lentjes was not opting for an operation and would just rehabilitate the injury.
He picked up the injury in the final game of the Highlanders season.
Halfback Josh Renton, who has a broken thumb, is touch-and-go whether he will be back by the end of the season.