Inter are struggling this season because they sold Samuel Eto'o this summer, says Alessandro Altobelli.
The Nerazzurri released the Cameroon ace to Russian club Anzhi for a reported fee of €27m, thus depriving themselves of a player who had netted 53 goals in 102 games for the club.
"Eto'o is one of the strongest players in the world," the former Inter striker told the Corriere dello Sport. "He can't be replaced.
"His exit is a problem for Inter because Eto'o, especially last season, covered up their problems with his goals.
"Without him those difficulties have surfaced. Someone like him made a difference in Italy."
The 30-year-old signed a three-year deal with Anzhi thought to be worth around €20m in wages.
"I personally didn't agree with his decision," Altobelli, a 1982 World Cup winner with Italy, continued.
"He could have won a few more trophies at Inter after his triumphs already on the peninsula and with Barcelona.
"He's finished with football at the highest level by moving to Russia."
The San Siro giants have had a problematic start to the campaign by picking up just four points from five League games, but Altobelli thinks things could improve.
"Inter's struggles this term were to be expected too when you consider the injuries they have suffered.
"I'm curious to see how they'll do when Douglas Maicon, Diego Milito, Wesley Sneijder and Thiago Motta are all back in the team."