"I know Fernando and Kimi well, both on and off the track. They are excellent drivers but I fear, as a team, there will be conflict," Massa said in an interview in Brazil this week.
"I have told (Ferrari's management) to breathe deeply now, because breathing will be much more difficult next season."
However Ferrari president Luca di Montezemolo has dismissed such forecasts of doom, saying Alonso was kept in the loop about the team's desire to sign Raikkonen and was happy with the choice.
"We are not masochistic enough to take on a driver without informing Alonso," di Montezemolo said in an interview with Gazzetta dello Sport.
"Fernando was always in the picture regarding the choice of Raikkonen, taken partly because the alternative, that of employing a youngster in what will be a complex 2014 season, did not inspire confidence.
"Today, Raikkonen is one of the best, along with Alonso, Vettel and Hamilton, and Alonso is the first to be happy that he is coming here."
The Ferrari president likened Raikkonen to former great Niki Lauda, who left F1 for two years and then returned to win his third drivers' championship. Raikkonen also had two years away from the sport before returning in 2012.
"The break has been good for him, he has returned to greatness, he's won races, he's finished lots of races," di Montezemolo said. "I wanted a driver who wouldn't make me look back on Massa with regret and I've got one. I want more victories, consistency, podiums from Raikkonen. Alonso will be the first to benefit."
The Ferrari chief has also dismissed concerns that Massa will be demotivated or selfish in the remaining races of his Ferrari career, despite the Brazilian being quoted as saying: "Starting on Friday in Singapore, I work only for myself."
Di Montezemolo described Massa as a "wonderful person" who will finish the season well.
"Supposedly he doesn't want to help Fernando? Nonsense -- he definitely will do it."
On the track, Ferrari's chances look slim in Singapore. If the team could not beat the Red Bull of Vettel on the high-speed tracks of Spa-Francorchamps and Monza, it stands little chance on the twisty street circuit of Marina Bay, where Vettel has won the past two races and where the Red Bull thrives in high-downforce settings.
Vettel is a strong favorite for Sunday's race, with the only potential wild card being Pirelli's selection of super-soft and medium compounds as its two choices this weekend, skipping soft and thereby creating intriguing strategy options.
"It's one of the toughest races of the year to be honest," Vettel said.
"It's a very long race, the full two hours, so the race just seems to go on forever. The circuit itself is a killer because there are so many bumps, there's no room for mistakes."
The 'Singapore Sling' chicane, where cars were regularly launched into the air by aggressive curbs, has been removed for this year, taking away one of the trademarks of the race but producing a faster lap.
