Cassano had helped inspire Samp to reach the top of Serie A in the opening weeks of the season but two 3-0 defeats in a row have coincided with a drop in form for the 27-year-old.
"He's down because he expected a reason for not being called up," General Manager Giuseppe Marotta told reporters. "Now he has understood it's written he won't go to the World Cup."
Sunday's home game against in-form Roma, now level with Samp just a point below fourth-placed Parma, offers Cassano the perfect chance to rediscover his form against the side where he spent a fractious spell between 2001 and 2006.
Roma, who could be without injured defender Philippe Mexes until January, are in confident mood after last weekend's 1-0 derby win over lowly Lazio, whose season is going from bad to worse.
Failure to win at home to Champions League-chasing Genoa on Sunday would leave coach Davide Ballardini on the brink.
The Rome side could therefore do without the distraction of an ongoing legal wrangle over the status of top striker Goran Pandev, who has been banished from the team all season alongside midfielder Cristian Ledesma after they asked for a transfer.
A special tribunal has been set up to see if Lazio have broken their contractual agreement with Pandev by not playing him and an imminent ruling could allow the Macedonian to move for free in the January transfer window.
Champions Inter Milan are a possible destination for Pandev as they will be without Cameroon forward Samuel Eto'o for the African Cup of Nations early next year.
Media reports have also said Luca Toni, barely used by Bayern Munich this term, is another potential solution, although club owner Massimo Moratti has hinted he will not buy.
Leaders Inter, whose advantage was cut to four points after Saturday's defeat at Juventus, travel to struggling Atalanta on Sunday where coaches Jose Mourinho and Antonio Conte are both banned from the dugout after being sent off last weekend.
Inter's Walter Samuel and Dejan Stankovic are likely to join their manager in the stand because of injuries suffered in Wednesday's 2-0 win over Rubin Kazan, which put the Italians into the Champions League last 16.
Second-placed AC Milan, also through to the Champions League knockout stages, are likely to be without injured defender Thiago Silva for Sunday's visit of Palermo.
Juve, in third, have a tricky trip to Bari on Saturday.
