De Gea admitted to the error after the match, telling reporters in the mixed zone: "I am fine, I am calm".
Both sides will still be heavily fancied to qualify from Group B and this match was essentially a battle between who will finish first and second.
They grew in confidence after signs of nerves early on and Isco and the old master Andres Iniesta started to pull the strings.
"Finding yourself in the opening game of a World Cup with a penalty against you in the second minute, you have to leave with good feelings after how the match went".
He tasted glory at last with his national team at Euro 2016 but was forced off in the final.
With the Sochi crowd on the edge of their seats, Ronaldo delivered with a trademark curler that bent over and away from the wall to settle a contest that will go down in history as a World Cup classic.
The game was full of chances, but was also a cagey affair.
With Portugal trailing 3-2 and the game entering its final minutes, Portugal was awarded a free kick from just outside the box.More news: Farm Bill Looks to Protect Dairy Farmers in Minnesota
Hierro was clearly happy after his first match in charge as his team showed they should by no means be written off.
The Spaniards sacked manager Julien Lopetegui, following his decision join Real Madrid at the end of the tournament.
Sporting director Fernando Hierro, a former player, was picked as Lopetegui's replacement and was on the bench despite no significant previous coaching experience.
Ronaldo was brought down by his Real Madrid teammate Jose Ignacio Nacho.
There is less drama leading up to this game for Portugal, which may work out in their favour on the pitch.
Cristiano Ronaldo made it clear that he means business at this World Cup after his stunning hat-trick secured a 3-3 draw here with Spain in a tournament classic and saw him reach another career landmark.
Portugal: Patricio; Soares, Pepe, Fernandes, Guerreiro; B.Silva, Carvalho, Joao Mario, Martins; Ronaldo, Silva.
A long Spanish free-kick was headed back across goal and the firebrand striker was in the right place at the right time to bundle home another equaliser.
Though Spain have been in impressive form in the lead-up to the showpiece event, they were hit by the shocking axing of their head coach Julen Lopetegui.
No matter where he's playing, no matter against whom, the spotlight is always on Cristiano Ronaldo.