The most-optimistic return date of late May 2009 would leave Evans with only a matter of days before the Lions leave for South Africa.
"There is cruciate damage and we are looking at an operation in the next couple of weeks, and then between six and nine months (out) after that," said Wales team manager Alan Phillips.
"It is quite a blow for us. He was starting to play well, so it is very disappointing for him."
"Ian will be in recovery for a period of six to nine months once he has had his operation," confirmed Wales physio Mark Davies.
"The injury he has is quite common and has good recovery rates, but unfortunately there is no quick way back and Ian will not be back in action this season."
Evans currently has 16 international caps to his name and had started against the Springboks and the All Blacks in the ongoing autumn series.
The latest setback is the second time Evans has faced a substantial spell on the sidelines following a 10-month absence with a shoulder injury that began in December 2006.