It was the "we need your legs" comment from Jurgen in meeting him that was surprising when you see him play.
It was like Steptoe had stepped on the field vs Newcastle. It was anything but legs. It could well be the German league makes you look quicker (he looked it on his YouTube footage, plus Keita looked much more powerful/quicker in that league). The first showing various people suggested he was tired having just signed, that was the hope anyway, but then you saw him again and he really is that slow. When he went off at Newcastle the tempo immediately ramped up, and we overturned a game in which we had been dominated. As you'd expect he has improved in his understanding of the team since then, and hes tidy, but the lack of athleticism and the effect he has on the 8s who have to stay near him so as not to expose him, restricts the midfield.
I dont think he would get in a top 10 PL side (he definitely wouldn't get in the 14th place Palace side who targeted him twice), this league is just so demanding physically (though Atalanta also did it to us/him). We saw last season what happened with Fabinho's legs going, pretty much every opposition ran through our midfield at will. Southampton who were already relegated opened up us time and again, with a completely shot Fabinho trailing behind the runners.
We were despondent in the summer when the big DM deals fell through as we knew how vital that position was. When Endo's name came up there was almost an incredulity (lots of googling), then Jurgen's first comments to him, which sounded promising (I thought this fella was going to be a Ji Sung Park type who could run all day). Its then when you see him, and the lack of mobility, and the restrictions it places on the midfield, when reality set in. The midfield has been disjointed all season as there isnt enough running power in the DM position to track runners when teams break (saw it again multiple times today, including when Bowen breezed past him which ended up with Allison saving, and when Kudus went past him which saw him get booked for dragging him back). The passivity, the lack of speed, athleticism, intensity, the lack of screening the back line, the slower tempo he imposes on the team, the 8s having to play closer to him as they know he doesn't have recovery pace when teams break (and being compact is not suited to a pressing side which takes risks), the effects are wide ranging. The 6 doing his job allows the 8s to do theres, and the whole team functions better, attacks are more effective with the 8s further advanced, and the defence has someone protecting them, putting out fires, preventing easy chances being given up (and we give up easy chances aplenty with a dreadful United putting 6 past us in 2 games despite us dominating possession).
It's such a key position that we have now gone two seasons without an adequate one for what the club aspires to achieve. It's the first thing Slot has to address as no doubt Klopp would have done.