Iniesta scored the opener from the penalty spot as Martino's men cruised to an easy victory that maintains their unbeaten start to the season, despite the absences of Lionel Messi and goalkeeper Victor Valdes.
Cesc Fabregas doubled the hosts' lead with another spot kick, before Alexis Sanchez and Pedro wrapped up the victory against Granada's 10 men.
But it was Iniesta who Martino singled out for praise, lauding the Spain international for his versatility.
"He's been the best player on the pitch," Martino said.
"When you see the game that he played, he can clearly do everything."
Martino made a surprise selection by including 17-year-old Adama Traore in the squad, the attacker coming on for his debut appearance as he replaced Neymar seven minutes from time.
And the Argentinian feels that the teenager has the qualities necessary for a place in the first team.
"It is not normal that a 17-year-old has the ability to play in the first team," Martino added.
"We believe he has the qualities to be here.
"Neymar returned from a long trip, and it was worth giving him a few minutes."
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