The Premier League club also have an option to make the move permanent at the end of the 2021-22 season.
"Alvaro is a player with huge potential, and we are very pleased that he has come in to our squad and our goalkeeper unit," said Brentford head coach Thomas Frank.
"He will come here and compete every day with David Raya and Patrik Gunnarsson. This is a signing that make us stronger as a group and will help push us to a higher level."
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