The Brazilian was yet to join the club when he sent a message to his agent while watching his prospective new club get battered by Brentford 4-0, and he has delivered exactly what he said he would.
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United began talks with Casemiro in June, with the midfield general keen to leave Real Madrid for a new challenge, however, there were fears inside the club that their nightmare start to the season - having also lost their opening game 2-1 to Brighton - might see the star change his mind.
But apparently it didn't. In fact, it appeared only strengthened his resolve that he was the man to help Erik ten Hag transform a spineless United side back into a team worthy of challenging at the top again.
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According to the Daily Telegraph, Casemiro was sat watching that miserable defeat to the Bees at his old home in Madrid when he texted his agent: "Tell them I'll fix it."
A team of United representatives then flew to the Spanish capital a few days later to seal the deal, a £60million transfer, and he has been working hard ever since to fulfil his vow.
That fee now looks a brilliant bargain, when you see how he has changed the United team.
Casemiro has been the catalyst to a huge change in fortunes for the Red Devils, who have recovered from their horrendous start to win six straight games, move up to fourth in the Premier League and go through to the semi-finals of the Carabao Cup.
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It was a slow start for Casemiro at United, with Ten Hag easing him into the first-team gently and it taking eight weeks for the Brazil international to make his first league start, but he hasn't looked back since.
United make more passes and concede fewer shots with Casemiro on the pitch.
His impact is clear when you give the numbers a glance, with United conceding 0.6 goals and 8.4 shots on average per game with Casemiro in the team, and 1.5 goals and 11.5 shots per game without him.
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But it's not just his ability to marshal the midfield and protect his back-four which is benefiting Ten Hag, with his passing stats all boosting United up the other end of the pitch, too.
United are averaging 64.1 passes into the final third per game with Casemiro in the team, compared to 51.3 without him.
This comes as little surprise, though, when you consider he played alongside Croatian maestro and Ballon d'Or winner Luka Modric during his career at the Bernabeu.
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In Casemiro, United also have a model professional and an example to young players - a dream for Ten Hag after he was forced to make an example of Cristiano Ronaldo.
It is claimed the Brazilian is one of the first to arrive at Carrington and one of the last to leave, he studies his opponents in great detail to prepare for every game, he regularly watches matches in his spare time and he is also having English lessons to improve his communication with his teammates.
That doesn't appear to be a major issue, though, with the Telegraph claiming United players regularly go to Casemiro for advice, and those at the club have been blown away by the time he affords his peers.
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After losing their first two games, United have lost just two of their next 15 fixtures - including a 6-3 mauling at the hands of Manchester City.
That game came during that eight-week bedding-in period for Casemiro, and you can bet it will be a different game when they meet again in the Manchester derby on Saturday, with the Brazilian now one of United's first names on the team sheet.
With the midfielder turning 31 in February, the only shame is that he didn't become a Manchester United player sooner, with the Red Devils finally having the world-class holding midfielder they have craved for years.
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