The Brazil fullback signed on-loan for Gremio this week.
He told the London Evening Standard: "I am not going to lie and say I was not disappointed.
"This was a last-minute deal and a clear message to me that I would not start for the rest of the season
"Monreal is a good player and he will be able to help Arsenal. However, since I play in the same position as him, yes I was disappointed."