The Welsh stopper left the pitch on a stretcher during Tuesday's creditable draw at The Stadium of Light against Sunderland.
A scan on his knee has revealed that he has torn a medial knee ligament and will have to go under the knife to cure the problem.
Weston will be out for at least a month and it will be a huge blow to him as it should rule him out of Wales' Euro 2004 play-off against Russia.
It is also a further injury problem for Bluebirds boss Lennie Lawrence, who already has Jason Bowen, Alan Lee, Peter Thorne and Scott Young on the injured list.
