Tomkins picked up the problem in the dying stages of England's narrow defeat to New Zealand on November 16 as his third rugby union cap ended after 77 minutes at Twickenham.
"Joel has got a knee injury, and will be out for quite some time, probably three months," said Saracens director rugby Mark McCall, after his side returned to Aviva Premiership action with a 16-9 win at Exeter Chiefs on Saturday.
"He picked up the injury in the last five minutes of the game against New Zealand and it is a maximum 12-week injury."
That length of absence would rule the former rugby league star out of all four Heineken Cup games left in Pool 3, with the first leg of the December double header with Zebre less than a fortnight away and the final group game at home to Connacht only eight weeks from now.
Sarries currently sit second in the standings, having won 23-17 in Galway in Round 1 before being edged out 17-16 by four-times continental kings Toulouse in a thrilling battle at Wembley a week later. A maximum-point haul against the Italians would keep their hopes very much alive prior to the trip to France on the weekend of January 10/11/12 but losing Tomkins is a big blow.
Tomkins has featured in both European encounters so far this season having missed out just once in eight Heineken Cup ties last term as Sarries reached the semi-finals before going down to champions-in-waiting Toulon in London.
The 26-year-old has been in fine form in 2013 and was called up to the England senior side for the recent autumn Tests, where he started all three games against Australia, Argentina and the All Blacks.