If you think Koeman knew who Onyekuru, Sandro, Vlasic and Gana were I think you'd be mistaken. It's obvious that Martina, Stek and Schneiderlin were Koeman signings. It's likely that Klaassen was also. Koeman also seemed to have a massive hard-on for Rooney and Sigurdsson. Who knows with the others? Yes, it's guesswork but educated guesswork.
You can go back all last Spring and see evidence of the Rooney hard-on. Perhaps he's the classic "starfucker" type manager (after all, he was a legend, and no doubt remembers what it was like when he was on the decline, likely thought he was owed deference then too) and has lost the dressing room.
Just a working theory.
I would personally give Walsh some time before I blew his reign up, he obviously had no hand in picking Koeman. Then again, no idea what @kramer0 is referring to re these rumours, so not dismissing the "clean slate" option, either.