Which manager signed him?
Neither manager. It was the club that signed him. He arrived a day after Bruce signed but the deal was in place well before that. Obviously Bruce had no input. Ashley said in his daily mail interview in July 2019 that they could've had the deal done as early as February 2019, but Benitez didn't want him. He wanted Rondon made permanent instead.
The club were the ones who had pressed ahead and made this transfer