The Pompey midfielder has been ruled out for 12 weeks following Carl McHugh’s tackle in the second leg of the play-off semi-final against Plymouth.
The heavy challenge left Evans with medial collateral damage (MCL) in his left knee and torn medial ligaments in his left ankle.
McHugh was booked for his part, while Evans was unable to continue and substituted after 31 minutes.
The Pilgrims went on to book a place in Monday’s League Two play-off final through Peter Hartley’s stoppage-time winner.
Evans now faces a summer of rehabilitation ahead of his second season at Fratton Park.
However, with the League Two campaign to kick off on August 6, at present it is unlikely he will be available for consideration.
The injury is a cruel blow to the energetic midfielder, who has impressed since arriving from Fleetwood last summer.
He has gone on to make 47 appearances and score 10 goals, collecting the Sky Bet player of the month for December in the process.
Only Marc McNulty (12) and Conor Chaplin (11) netted more times for the Blues this season.
In February, Evans earned an additional year’s stay having activated a contract clause upon reaching 23 league starts.
The 28-year-old was not alone in suffering injury during the Home Park defeat.
Danny Hollands was forced off with ankle ligament damage, also during the first half.
He is expected to miss at least six weeks with the problem.