However, Root said that since the veteran opener has already made up his decision, there was no point trying to change his mind.
“I was a little bit surprised but he was adamant in his decision. You could see in his face that this is what he wanted. There was no point in me trying to talk him round,” Root said.
Cook was presented with a commemorative cap by Andrew Strauss and Colin Graves before his final Test. He made 71 in the first innings on Friday.
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Root, who took over as England captain from Cook last year, spoke of his sadness when the all-time national record runscorer took him aside last week to explain his 161st Test will be his last.
England subsequently sealed the Specsavers series with a 60-run success to go 3-1 up with one to play, and Root is hoping the stage is set for both Cook and his team to deliver the perfect send-off.
Root confirms the veteran opener is not the type to lap up the fuss, but reasons it will be entirely appropriate on an occasion such as this – and a 33rd Test century to match his age would not go amiss either for a batsman who began his “fabulous career” with a hundred on debut in 2006.
“It would be nice to start and finish with a century – you never know, it might be written in the stars,” he said.
“He doesn’t want too much fuss – but if ever there was a time for it, it is now.
“It means a lot to the whole dressing room, and they will be desperate to do everything they can to give him a great send-off, that means making sure we win this game.
“It’s an extra incentive for us as a group and would be a fitting way to finish a fabulous career. He just deserves a fantastic week, and I hope we can deliver that for him.”
Root feels a particular personal debt of gratitude.
“He’s been a huge influence on me,” he added. “As a role model, there aren’t many people who go about things in a better way.
“He’s the most down-to-earth superstar you’ll ever see. He’s always got time for everyone.
“That’s a great quality to have in any walk of life, never mind a high-pressure environment like international cricket.
“I’ll miss him being around – not just for his runs and what he adds to the team but as a person as well.”
Cook singled out his captain as confidant about his impending retirement before the fourth Test.
Asked what his reaction was then, Root paused noticeably before saying: “Sad. It’s been an outstanding career – 12 years at the top of the order.
“There’s not many openers around the world who can match that, never mind English players.”
He added on BBC Test Match Special: “The way he conducts himself in the dressing room, to every single person, the respect he has for the game, for all of his team-mates and the opposition, it is what everyone aspires to. You’ll never see someone of his stature again in the game.”
Root had one other significant matter to address at his pre-match press conference – the unexpected decision to hand England’s Test wicketkeeping gloves back to Jonny Bairstow, recovered from the broken finger which meant Jos Buttler did the job instead last week.
Root insists there are “no issues or problems at all” between his fellow Yorkshireman and Lancashire rival.
“Jonny has had the gloves for a long time and done exceptionally well for a good period of that,” he said.
“I think he deserves the opportunity to keep wicket in Test cricket. I also think Jos has done exactly the same in white-ball cricket, why mess with something that has worked so well for so long?”
Jonny Bairstow will regain England’s Test wicketkeeping gloves from Jos Buttler at The Oval after proving he has recovered sufficiently from his broken finger.
The switch comes after Bairstow had to play as a specialist batsman against India in Southampton last week while the finger he broke previously at Trent Bridge was still healing.
England completed a series victory over India with a 60-run success at the Ageas Bowl, and it was thought highly likely Buttler would retain the gloves for the final Specsavers Test of the summer.
It was Bairstow rather than Buttler who was busy with his wicketkeeping drills, however, as England prepared at The Oval over the past two days.
Then shortly before captain Joe Root addressed an early-afternoon press conference on Thursday, a statement on the England and Wales Cricket Board website read: “Jonny Bairstow will keep wicket after recovering from a broken middle finger.”
On the same topic, Root added: “I hope (Jonny) uses it as a real driving force to keep improving.”
Root also spoke about the moment England’s all-time record runscorer Alastair Cook informed him this Test would be the last of his career.
Asked for his reaction when Cook confided in him last week, he paused before saying: “Sad.
“I’s been an outstanding career – 12 years at the top of the order.
“To have a record like that is some feat, especially in these (English) conditions for the majority of it.”
England have stuck with the same XI which beat India in Southampton to take an unassailable 3-1 lead.