The thing is, if we think he's good enough long term, it would take a really serious injury to put us off buying him. Yeah you might get a few quid off but it would be peanuts over the life of his time here (again, if we think he's good enough). If he has an insane world cup that's the sort of thing that starts bidding wars with teams previously not in the picture, and really silly numbers being thrown around especially for wages.
We already own a bunch of players that could get injured this world cup. Getting this done before it starts is highly preferable, IMO.