The Poland defender has joined Porto on-loan for the season.
He recalled to O Jogo: "It was difficult to say goodbye to Arsenal, because I didn't know when it would actually happen. If I had known the exact day in advance, I would have had time to prepare.
"In the last three days there, I didn't know which would actually be my last. Fabrizio Romano launched the here we go campaign, and I started receiving dozens of messages and calls congratulating me.
"I was starting training. The players looked at it on their phones and asked me why I had equipped myself when it was already closed."
He continued: "I refused to say goodbye there because I still didn't have any information about what might or might not happen.
"Next day I knew I could say goodbye again. I went to the pitch to say goodbye to my teammates and the coach. They set up a farewell tunnel, and it was tough.
"Not everyone I wanted to say goodbye to was there, so I still had to make a few phone calls."