Mikel Merino spent several weeks injured earlier in the season, but his versatility has been vital in building up momentum that led to Arsenal winning the Premier League after 22 years. As Spain repeatedly came up short in their bid to prise open the Portugal defence, manager Luis de la Fuente sent in the tall midfielder as an 85th minute substitute.Portugal's Cristiano Ronaldo dejected after the match against Spain after his team got eliminated from the FIFA World Cup. (Reuters)Merino would perhaps have been a sure starter but for the re-build post injury, although he is a dangerous presence in the rival penalty box. Arsenal’s Mikel Arteta says his substitutes are more “finishers”, and Merino aptly fits that description.His strike in the first minute of added time sent Spain into the quarter-final (versus Belgium or USA) and sent Portugal out of the competition and its ageing talisman Cristiano Ronaldo finally into international retirement.The strong World Cup contenders found late passing momentum, after being a touch cautious despite possession and control as Portugal effectively marked Spain’s biggest threat, Lamine Yamal, on the right.With extra time looming, Portuguese substitute Bernando Silva took a yellow card for a foul on Merino. Silva targeted him again to break up a Spanish build-up halfway into the Portugal half.Merino re-started quickly and ran to the top of the box, and received the pass from Ferran Torres, and finally beating Portugal goalkeeper Diogo Costa at the near post. It was a deserved result, giving joy and relief to Spain. They next face winners of the Belgium-USA last 16 game in Seattle.Portugal had raised hopes with a few efforts, but none that could trouble Spain goalkeeper Unai Simon, who has now gone a record 609 minutes without conceding a goal in World Cups. Some of the late crosses were either too deep or lacked direction.It was an irony that Portugal’s best chance deep into stoppage-time as they chased an equaliser, came from one of the shortest men on the pitch. Bernardo Silva’s header went just over the bar. It was also a telling commentary of how Portugal, packed with some of their most talented players, were let down by the tallest player in the attacking third.Ronaldo, 41, signed off with 233 caps and a world record 146 goals for the country – In 27 World Cup games he scored 11 goals), but he was a shadow of the man whose hat-trick, including an 88th minute free-kick, held Spain to a thrilling 3-3 draw in the 2018 World Cup.On Monday night though, while his teammates were tireless in their mostly defensive duties, Ronaldo looked too slow to offer them any support. Of the two chances that fell his way, he flicked one without power to allow Unai Simon to recover and collect. Earlier, he didn’t take a shot from inside the box and the chance fizzled out.Ronaldo had announced on Sunday that this would be his last World Cup. The man who made his international debut as a teenager almost 23 years ago and built a legacy along with Lionel Messi, was left on the pitch by manager Roberto Martinez. A younger and faster substitute – Goncalo Ramos was an unused sub – would have put more pressure on the Spanish defence.Official TV cameras surrounded Ronaldo, who stood with tears in his eyes. In an image that offered the future of football, 18-year-old Yamal walked up to hug the Portuguese icon. He waved to the crowd before walking away in silence.Well-drilled SpainAlthough Spain have been slick in their movements and passing, it was a slow start and surprisingly missed early chances. Forward Mikel Oyarzabal, perhaps surprised that the off-side flag stayed down, sent a left-footer wide with only Costa to beat in the eighth minute.Left fullback Nuno Mendes was outstanding in marking Lamine Yamal – the youngster found more freedom only after Mendes was taken off injured in the second half. Still, he produced the best chance in the 16th minute, but Costa made a double save, punching away Yamal’s attempt and then getting his finger tips to Alex Baena’s shot off the rebound.Portugal took better control of the midfield, but Ronaldo looked every bit like a man finishing his career. He didn’t go for fifty-fifty challenges and even his footwork seemed to lack energy. There were occasional passing moves by Portugal but the final delivery seldom came.Close to halftime, Nuno Mendes’ fierce shot from the corner of the box was headed on to the horizontal by Pedro Porro as it bounced back into play. Pedro Neto was guilty in the second half of not taking a shot and instead sending the pass across the goalmouth with Ronaldo still catching up.Spain will be glad to come through a tough game against a familiar rival, but Portugal are set for a rebuild without their playing icon as well as manager Martinez, whose assignment was until the World Cup.