for this situation, I think we can compare Zinadine Zidane and Ryan Giggs. When Zidane manage Real Madrid, in the first year he get a champions league with that players. And now with manchester united play in under champions league competition called Europa League, with their squad now they can win the europa league with Ryan Giggs.
Before taking the head coach position in Real Madrid, Zidane was a coach of Real Madrid Castilla and became coach assistant for Ancelotti in senior team. So comparing to Giggs, Zidane had more experience, thats why he got success in his first time as head coach for Real Madrid. But who knows, maybe Ryan Giggs could surprise us.
Well we know Arsenal is a big team but they dont win Premier League for years, and Arsene Wenger still there. It totally depends on how patient the fans supporting their team.
also Zidane got a team full of stars and it only needs good management not like Manchester united because united are far from being a team that can achieve something we can see that for the past three tears they always been poor and Madrid had great results but didn't win trophies so it's difficult to compare both teams situations.