The No 8 left Murrayfield in crutches on Friday night after injuring his knee in the first half of the win over Benetton.
Edinburgh are awaiting medical reports but the initial prognosis was downbeat and Mata will play no part in Saturday’s European Challenge Cup clash against Saracens or the 1872 Cup double-header with Glasgow Warriors.
“Not the best of news,” acknowledged Mike Blair, the Edinburgh coach. “It is a big blow for Bill himself and the club.”
Winger Damien Hoyland has also been ruled out of the festive fixtures with a knee injury and there is a question mark over stand-off Blair Kinghorn’s availability to face Saracens due to a head knock.
But it is Mata’s absence which looks the most serious. The Fijian international has been a key performer this season and ranks sixth in the United Rugby Championship for carries after starting in six of Edinburgh’s seven games.
Blair said Mata sustained the injury in fairly innocuous circumstances during the 24-10 win over Benetton.
“We had a five metre scrum and he picked up and got caught awkwardly. Nothing malicious,” said the coach.
With a tough game looming against Saracens, Blair stressed that Mata’s misfortune could open the door for someone else.
“It gives other guys the chance,” he said. “We are pretty fortunate with the coverage we have in the back row so we will give another player another opportunity to grasp that jersey.”
Magnus Bradbury and Nick Haining are likely to come into contention for the No 8 role at the Stonex Stadium.
Hoyland misses out after injuring his knee against Benetton.
“He has had a scan on his knee and we are waiting to get the results back on that. He is not going to be back for the weekend,” said Blair, who added that Hoyland would also miss the games against Glasgow on December 27 and January 2.
Kinghorn, meanwhile, went off in the closing minutes against Benetton for a head injury assessment.
“He is going through the protocols and we will see over the next couple of days,” said the coach.