The news will be a bitter-blow for the exciting attacker who spent nine months on the sidelines after suffering a multiple fracture-dislocation of the same leg in April last year.
Thomas hobbled off to pitch during yesterday's league encounter against Benetton Treviso and is set to undergo surgery.
Dragons rugby manager Robert Beale said: "We are bitterly disappointed for Martyn. He's had a hard road to recovery from his previous injury and this is a heavy blow for him.
"However, he's shown great strength of character through his rehabilitation and we are confident that he will meet this next challenge head-on."
The Dragons will return to league action without Thomas on April 15, when they face the Ospreys in a crunch Welsh derby.
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