I pray and hope he declines the offer and stays with you lot. If he has any sense he would do just that. Stay where he is, he has nothing to prove, he has won the Premier League, will be playing be Champions League, he is a club legend, gotten into the England Squad, basically can't do any wrong... Why throw all that away for a few extra grand and potentially get all the abuse if things don't go well at the Emirates. Fans are very fickle (particularly gooners)! If I was his adviser, I will be advising him to stay put.
Look at Walcott/Ox one minute they were the future of Arsenal, now they are almost like pariahs. He is out of form at Leicester, your lot will nurse him back to form, however at the Emirates, he would be booed and jeered.
... couldn't have put it better myself ... unfortunately I doubt his agent (or missus) will see such sense and will want to cash in whilst they can ...
... and we all know your training regime will have him out for half the season to boot

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