Merino had just been knocked down and a foul was called. While a Portugal player argued, he played the ball back in, ran towards goal and easily beat goalkeeper Diogo Costa after taking a pass from fellow substitute Ferran Torres.
Spain advanced to the quarter-finals for the first time since winning their only World Cup title in South Africa 16 years ago. La Roja will next play Belgium, 4-1 winners over the United States later on Monday (Tuesday AEST), on in California on Friday.
"He's one of the best in his position worldwide and he has given us a fantastic result and a fantastic goal," Spain coach Luis de la Fuente said through a translator.
"I want to express the importance of substitutes that came in later in the game. Not just today, but the contribution in other games has been enormous."
Ronaldo was trying to get Portugal to the quarters in a second consecutive tournament for the first time. Instead, the career on soccer's biggest stage is over for the all-time leader in international goals (146) and appearances (233).
Merino didn't enter until the 85th minute and his heads-up play showed some of the versatility that helped Arsenal win their first Premier League title in more than 20 years in May. He had been doubtful to make Spain's squad because of injuries that also impacted his Premier League season.
"Yes, this is my last game with the (Portugal) national team," said Roberto Martinez, a native of Spain who turns 53 on July 13.
"I'm proud. I've had 45 games, and I felt welcomed in Portugal and loved.
"We didn't fail ... we lost a game against a team that's one of the favourites.
"It's hard, but it's the end of a cycle and in the context it makes absolute sense ... It's important to have a new voice now."
The latest meeting of Iberian Peninsula rivals that first played in a friendly in Madrid 105 years ago was quite the contrast to their most recent World Cup match.
It was eight years ago that Ronaldo had his only World Cup hat-trick in a 3-3 draw with Spain in a group stage opener in Russia that is considered one of the tournament's best games.
"I'll wake up tomorrow like I woke up today, with a clear conscience," he said.
"I gave my best. I won three titles with Portugal. Before Cristiano, Portugal hadn't won any title. So, I'm happy. The biggest title that I won with the national team was in 2016 (European Championship), which for me had the same dimension as a World Cup, honestly."
The 41-year-old superstar scored three times in this tournament, but didn't have many chances against Spain's Unai Simon, who extended his World Cup record to 609 minutes without conceding a goal.
In the process, Spain became the first team to record six straight shut-outs at the World Cup, breaking the tie with Italy (1990) and Switzerland (2006-10) - a streak which began in 2022.
With Reuters.
5°C