If we lose and he is thinking of leaving, I think he will announce it after the game.
If we lose and he doesn't announce it, we can assume he is staying.
As I said, losing against a club in a worse position than us, with a manager under even more pressure, would mean this is the worst run of games Wenger has had with us.
It would also mean more discontent with the supporters and the players, which might be shown at levels we have never seen before, and I don't mean a few planes with banners trailing behind them, or some Wenger out posters outside the ground.
Even the most ardent Wenger supporters will start to realise we need a change, so winning this game is the bare minimum he needs.