There's no getting away from the fact he has only won 1 title. Even throughout his managerial career he has only won 3 over a 23 year career and 1 champions league. I'm sure Klopp would of liked 5 titles at 74 points, rather than 1 at 90 points.
That's a poor return, for people to claim he is an all time great manager. That should be for the likes of paisley, fergie, capello, mourinho etc etc. The greats win titles, year in year out. They find a way to conquer new challenges and rise to it.
One manager that is underrated is Ancelotti, now that guy is an all time great.