Look, you were born in 1994. I'm just gonna tell ya that there was no season that Beckham or Giggs were more important than Keano unless you're talking about when he was injured and we didn’t win the league, because that's what happened.
You need to give yourself a history lesson. Start with the career of one Roy Keane.
Yeah I think you need a history lesson if you genuinely think Keane was undisputedly the best player for 10 years in England. It's just not true. At all. He had years that yes he was. But he was not seen as so, undisputedly. Maybe in your eyes. But if it was undisputedly, then he would rank above United players in ballon D'or rankings every year. Which wasn't the case.
Undisputed best would mean Arsenal fans accepting he was better than Vieira (not the case). Undisputed best would mean that there wouldn't be multiple premier league players ranked in the Ballon D'ors every year, when he wasn't... Or in 1999, where Beckham was ranked #2 and Keane in his biggest year was #6. In fact Keane was only in the top 30 twice, in 1999 where he was 6th, and then he got 1 vote in 2000. While Beckham, Giggs, Schmeichel, Cantona all got multiple years of good rankings. Let alone other PL players. Call Ballon D'or rankings what you want, but your statement is a pile of horseshit and you know it.
Anyway - like I said. Keane was fantastic. Casemiro is fantastic and IMO as similar a player as there has been to him in modern football. It's fair to compare them - and it's fair to say that in terms of achievements, Casemiro has surpassed pretty much any United player.