A better question is who was the last Ajax manager to seriously impress on this scale!
- Before Ten Hag they went 4 years without a title win, with Ten Hag they have won the league every full season he has been there (well 19/20 they were first and then the season was null and voided).
- They hadn't reached the knockout phase of the CL since 05/06, though they finished 4th that year in the league. Before that was 02/03, but again, weren't champions. Before that was the 1994/95 - 96/97 team under Van Gaal where they won it once, were runners up once and then semi finalists once with 2 title wins in that time too
- They hadn't gotten a league and cup double since 2001/02, while with Ten Hag they have done it twice in 3 seasons and might do it this year
- His 2018/19 team with Ajax scored 119 league goals, only bettered 3 times in their history (85/86 with 120 goals, 72/73 with 120 goals, 66/67 with 122 goals).
- Their goal difference in 18/19 of +87 was only bettered twice in their history, and they've averaged 3 league goals per game since he took over and conceded roughly 0.75 goals per game
I think it's just a case that no Ajax manager has had this level of success since Van Gaal in the 90's, and in general only the greats like Michels, Cruyff, Van Gaal, Kovacs.
On the flip side before we get carried away, in terms of league title wins at least, Frank De Boer did well in the league with Ajax and won 4 in a row, however he flopped in the cups and the CL got eliminated in the group stage every time, and his league domination stats weren't as impressive.