There is no 100% way to judge a manager moving to a new team same as a footballer.
To use your example just because Kezman or Huntellar were flops doesn't mean you can't find a Ruud or Suarez from that league. I had a personal short list of three (Zidane, Conte, ten Hag) and believe we should do anything within our power to get one of them. We've already fecked up Conte, so need to go all out for the other two.
Here is Klopp talking about Ten Hag:
https://www.liverpoolecho.co.uk/spo...rpool-manchester-united-jurgen-klopp-22117685
I will also add there are no guarantees in football but two years of the likes of Rashford, Greenwood, Hannibal etc getting high level coaching and the fans getting to see high energy attacking football will be worth it. If he doesn't win anything after two years then sack him and try a new guy. It's really not that complicated.