Ramsdale dropped a clanger as Yoane Wissa struck an equaliser before Kai Havertz's late winner for the Gunners.
Frank was seen chatting with Ramsdale after the final whistle and later revealed: "I think I actually said to Ramsdale I don't think he deserves to be criticised for a mistake.
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"I think it's more unbelievable pressure from Wissa. If you see the way he accelerates it could happen to David Raya.
"I think it was fantastic pressure from Wissa and if you saw how many times they went long every single time we were very brave to go man to man."
Frank added: "I said I think he's a very good keeper and I didn't want him to be involved in that situation.
"Again I can't say strongly enough I don't think it was a mistake, but very good pressure from Wissa."