Isn't nor ever will be nearly as good. Replacement for Fernando maybe, but even he has played at CL level for many seasons.
Delph plays for Villa at 25, a team that escaped relegation by 3 points. As with any transfer for a clearly sub par player, the more it looks like it's actually happening the more desperate posts start to look, and the more that player can 'do a job'. This eventually turns into them becoming 'potentially world class', usually right after signing, descending into 'Shit, sell him to Sunderland', as soon as he's actually played a game. It's like the scouse boom bust cycle but with players instead of the league.
It's an 8 million quid punt in the event everyone else is injured. Lets not kid ourselves that it's anything but.