Let me list, just from recollection, the players Poch wanted versus the players he got.
2014 - wanted Hector Moreno from Espanyol. Got Fazio. Wanted Morgan Schneiderlin from Soton . Got Benjamin Stambouli. Wanted Jay Rodriguez, also from Soton. Got no one.
2015 - Wanted Berahino. Got N'Jie. A failed move which ruined Berahino's career, incidentally.
2016 - Wanted Mane, from Soton. Got N'Koudou, after he spent 40 days in a hotel waiting for Levy to agree a fee. Wanted Wijnaldum from Saudi Sportswashing Machine. Got Sissoko. Wanted Zaha. Didn't get him.
2017 - Wanted Barkley. Didn't get him, or anyone, for attacking midfield.
2018 - Wanted Grealish, eventually. Didn't get him. Got absolutely no one, in a dereliction of duty by the board that still haunts us today.
2019 - got who he wanted, as I understand it.
In all the years he was here, he was only ever backed once like Klopp was backed, despite taking us to the same heights Klopp took Liverpool. He had to make do with second-choices and bargain bins, and the funny thing is, he turned them into absolute stars more often than not, despite that. Out of all his signings, I can think of maybe six that failed - Fazio, Stambouli, N'Jie, N'Koudou, Janssen, Aurier. And out of those, roughly 2/3rds were the bargain bin rude boys that Levy used to pull out to provide warm bodies where needed - Fazio, Stambouli, N'Jie, N'Koudou.
Blame lies on him for Janssen and Aurier, both expensive and high-profile mistakes. Which, out of 26 signings (many not his first-choices) isn't half bad. There are some still in the 'iffy' category, including Sanchez, Ndombelly and so on, and if you really want to, you can count them against him too (although in that case, you also have to give him credit for Lo Celso from this summer - the polar opposite to the frustrating Tanguy). Other than that, I'm really not seeing where this precipitous decline in his ability to improve or identify players came from.
We had finished 3rd, 2nd and 3rd and then decided to become the first British team in the history of the modern transfer window to make no signings - *twice*.
Thst probably has more to do with the drop-off in energy, motivation, fitness and meteorology than anything else - utter stagnation, because of that dereliction of duty.