Athletic Bilbao stopper Simon produced one of the biggest howlers in recent memory to gift Croatia the lead in their last-16 thriller.
The 24-year-old, who has been given the nod ahead of De Gea for all four of Spain’s games so far this summer, failed to control Pedri’s back pass, with the ball rolling into the back of his own net.
The two sides headed in locked at 1-1 in Copenhagen thanks to Pablo Sarabia’s equaliser.
But Manchester United keeper De Gea waited for Simon while the rest of his team-mates poured into the dressing room, and they shared several words as they trudged towards the tunnel.
Simon was clearly explaining what had happened, and the more experienced De Gea listening diligently, before no doubt giving his fellow countryman some advice, which went a long way to securing victory for Spain.
The Pedri own goal kick started what happened to be one of the games of the tournament at Euro 2020, as a 3-3 thriller ensued in normal time to take the game into extra-time.
Spain eventually scored twice in the first half of extra-time to swing the tie their way.
But prior to that, Simon was on hand to produce a quite incredible save with his legs to keep the scores level after five minutes of extra time.
And he earned serious praise from Spain boss Luis Enrique and team-mate Cesar Azpilicueta at full-time.
Enrique said: “Football is a sport of errors sometimes. But after the error Unai gives a lesson to everyone who plays the game of how not to let your head drop.
“It’s normal that we show inexperience sometimes but what no one can doubt is that this team can score goals and play good football.”
Meanwhile, Chelsea defender Azpilicueta added: “We all make mistakes and Unai’s reaction was to do everything to make up for it.
“He saved us several times. You can’t ask anymore. Players are human and make mistakes.”
Graeme Souness admitted that Simon went from zero to hero thanks to his match-defining save in extra time.
“Croatia started really well in extra time, they had a couple of great chances,” Souness told ITV Sport.
“Goalkeeper makes an incredible save, you imagine that goes to 4-3 for them (Croatia), it’s a very different remaining period, that first half.
“The goalie should be a hero now – from being a villain, he’s turned himself into a hero.
“Saving that shot at that particular time has made the difference in this game.”