Manchester United boss Mourinho was in charge of Barca’s bitter rivals between 2010 and 2013.
The rivalry between the La Liga giants has never been fiercer than it was during that period and often prompted ugly scenes when the pair played.
Mourinho had a number of spats with Pep Guardiola when he was in the Barcelona dugout.
And Iniesta said the United manager was solely to blame for that when quizzed on the subject.
“You don’t have to be for Barcelona or Real Madrid to know that the situation was unpleasant,” he said.
“And the key component in that story was Mourinho. Whoever doesn’t want to see that, is radical.
“You didn’t see the rivalry that always existed before, it went beyond that, you saw hate.
“That atmosphere developed and it was unbearable.
“The Barca-Madrid tension caused by Mourinho did much damage to the national team [Spain] and the team-mates.”
Mourinho is under intense pressure at Old Trafford after guiding them to a dreadful start this season.
Following Saturday’s 0-0 draw at home to Crystal Palace, they are seventh in the Premier League table, seven points off the Champions League places and a whopping 14 behind Guardiola’s Manchester City at the top.
Mourinho continues to be a controversial figure and has drawn criticism from United legends for publicly calling out his players.
In a recent interview with a US-based TV channel, Mourinho questioned the character and mentality of Luke Shaw, Anthony Martial, Marcus Rashford and Jesse Lingard.
And Rio Ferdinand said the manager cannot expect the players to play for him if he continues to criticise them so openly.
He said: “Do you expect to have players on your side or do you expect your players to go ‘hold on a minute, my manager is hammering me in public again’?
“I just don’t think it’s right for a football manager to be doing that.
“I don’t think you’re going to get productivity out of those kind of comments. I don’t think it’s proactive for the club.”