The Kiwi hooker broke a bone and damaged ligaments in Saturday's Magners League win against Edinburgh at Rodney Parade.
He went under the surgeon's knife yesterday and has been told he will be sidelined for up to three months, effectively ending his season.
The 31-year-old has struggled with a chronic knee injury this season and has only made seven league appearances to date.
The news is a major blow to the Dragons, who have won their last three games in all competitions and are looking to climb the Magners League table with a strong finish to the campaign.
But Steve Jones has lessened the blow with his timely return to fitness following a shoulder injury.
Jones will battle for the number two jersey with Lloyd Burns and a host of Premiership hookers.
Willis was also banned last night for two weeks for stamping on an un-named Scarlets player during the Welsh derby at Rodney Parade on February 6 but his injury means he won't be affected by the punishment.
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