The result deprived the reigning champions of top spot in Group D, handing them a tougher route to the final in the knock-out phase as Italy await them in the last 16.
But it could have been so different had Ramos converted a second-half spot-kick, with Spain punished by the impressive Ivan Perisic's late winner.
"I was going to take it, but Ramos was confident and only the one who takes it can miss," Iniesta said in quotes reported by AS.
"The confidence is entirely in whoever wants to take it. We had the misfortune of missing.
"These things happen and now we have to continue to prepare for the game against Italy with the aim of going as far as possible.
"In theory we have stronger teams [to play] on paper, but you never know. These things happen in football and you have to be prepared for everything, not only for the times that you're being praised.
"We're on the more complicated side [of the draw] in theory, but we are Spain and we have to remain confident."
Italy, who play Republic of Ireland on Wednesday, are assured of top spot in Group E.