Romero struck a late winner against Real Madrid after earlier surviving a red card appeal after a challenge on Mbappe.
He admitted afterwards: "I knew it was impossible to stop him on the run, I tried to stop him as best I could. It was a bit of a nasty tackle, I apologized. It didn't go any further, it stayed there.
"I think this gives us life. We've been playing good games, competing. We knew we had to suffer, that it was going to be a full house. It was incredible, we have to thank the fans. These are three very important points for what's coming."
Romero also praised the performance of goalkeeper Joan García: "He deserves it. He's a great goalkeeper, he's going to be one of the best in Europe. He's fundamental for us, he gives us points, victories. The victory is more his than mine."