Messi has helped Barca to a decade of almost unprecedented domestic dominance, winning nine of the 14 Liga titles he has disputed since making his debut in 2004.
The Argentine has also lifted no less than four Champions League crowns with the Blaugrana, who had won just once in the competition prior to Messi's arrival.
And while he is ably assisted elsewhere on the pitch, Cappa believes that if he were absent Barca's success would have been impossible.
"Without Messi, Barcelona would be 10th. He's saved them in many games," the veteran coach, who assisted Cesar Luis Menotti during Argentina's 1978 World Cup win as well as during his spell at Barca, told Radio Mitre.
“Before talking to the coach and players, he’ll play on the PlayStation. Then, on the field, he wants to be the leader. He’s the best in the world with Cristiano [Ronaldo]," the 1986 World Cup hero had fired.
“For me, it’s difficult to say it but it’s useless to make a leader out of a man who goes to the bathroom 20 times before a game.
“There’s no two ways about it. Let’s stop making a God out of Messi. Messi is just another player for Argentina.”
Cappa agreed with his former charge's statements, stating that Messi was after all human.
"I agree. We must not deify Messi. He is a mortal player, the rest is marketing," he added, while expressing his hope Leo would return to international football.
"We must get him in again but it doesn't make sense to call him up for these games."
The Albiceleste take on Brazil in a friendly on Tuesday without the absent Messi, as caretaker manager Lionel Scaloni looks to help the national team recover from their shambolic World Cup campaign in Russia.
But Cappa does not believe Scaloni should be named as Jorge Sampaoli's permanent successor, instead remaining hopeful that Spurs boss Mauricio Pochettino could be tempted to return to his homeland ahead of Copa America 2019.
"Lionel Scaloni as coach? He does not have enough experience to continue in charge of the national team, I like Pochettino."