[EricH]

I'd take a ref at each end watching only the box (which is already in use elsewhere) over instant replay; but some people won't go for anything less than instant replay.

A goal-line monitor could work, but it wouldn't solve either of the two problems that have shown up (the hand-ball in the qualifiers, and now this). I'd go for explanation required if a number of players request it--nobody really knows what the call even was, except the ref, so even what was called is better than no explanation at all.

If that whistle is later shown to be a mistake, then I'm not sure there will be many more assignments for the ref for a while if he doesn't acknowledge it at his media session. Would you want a ref who made that high-profile of a mistake and didn't admit it? (Note that Jim Joyce was later voted as the best ump in MLB by the players, for admitting the mistake immediately and on his own accord.)