WTF, you could use that argument with pretty much every (successful) transfer that's ever happened in the history of the game.
With regard to convincing the club that we needed him, I'd say he probably did after Zamora's six goals in six loan games. Or are you going to say that Zamora himself...
FOUR seasons. He took over in November 1986 and didn't win anything until the FA Cup in 1990. Didn't win the league title, what he was brought in to do, for another three years. The rest, as they say, is history.
Something to be said for giving a manager a bit of time?