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It all happened during Atletico Madrid's 2-0 win over Juventus on Wednesday after the opening goal was scored.
While Juventus tried to preserve the stalemate, Atletico were bold. Simeone introduced Morata, Thomas Lemar and Angel Correa, all around the hour-mark, and his daring paid off.
Crashing out in the last 16 would be unthinkable for two clubs that were among the favourites when the draw for the knock-out stages was made in December.
While playing for Real Madrid, Ronaldo has scored 22 goals in 31 matches against Atletico Madrid, who are one of his favourite opponents.
The Argentine coolly dispatched a spot-kick just before half-time, drilling beyond the reach of Jordi Masip after Gerard Pique was bundled to the ground by Valladolid midfielder Michel.
Paulo Dybala returns to the starting line-up, forming an attacking trident alongside Mario Mandzukic and Federico Bernardeschi.
Real Madrid's fifth league win in a row means they moved above Atletico and are in second for the first time since September.
With Barcelona being held by Valencia, Atletico Madrid losing to Real Betis, Real Madrid's 3-0 win helped them gain ground.
Oscar Ortega was taken into custody late Monday and released after appearing before a judge on Tuesday.
The striker, who had a disappointing time at Stamford Bridge, passed his medical on Sunday, the Spanish side, his first club, said in a statement on its website.
Morata was shown leaving Atletico's medical clinic on Sunday morning, with the 26-year-old giving a thumbs up and telling reporters, "everything has gone well".
The country remains on thin ice with the governing organisation, currently sits 199th in the FIFA rankings and has still never qualified for a football World Cup.
The win was Atletico's first away from home in La Liga in nearly three months.
Monaco, which had arrived with no chances of advancing, missed a late penalty as former Atletico forward Radamel Falcao shot wide.
Barca produced another fightback as Dembele, on as a substitute, slid home a cool finish in the 90th minute to earn the visitors a 1-1 draw.