The left-back thinks his former club mate has joined the Bernabeu outfit and possesses everything needed to succeed in the capital
The young attacker is the subject of a transfer tug-of-war between los Blancos and Manuel Pellegrini's Manchester City, with both teams vying for his signature to rebuild following failed title bids.
And Monreal, who played alongside Isco between 2011 to 2013 at La Rosaleda, says his compatriot has snubbed Pellegrini's approaches for a move to the Bernabeu.
"Isco has succeeded wherever he has gone. I believe he has already signed for Real Madrid and I'm sure he'll do very well because he is a great player," the 27-year-old told Marca.
"He may cost them, but together they will have opportunities and they will seize them."
A likely victim of los Blancos' revolution will be Gonzalo Higuain who has been tipped to join Arsenal this summer, with Monreal welcoming the striker to the Emirates with open arms.
"I would love it. Higuain is a part of football, and I always loved how he played for Real Madrid. He is a scorer and would be a reinforcement that we could really use," he continued.
"Arsenal are one of the biggest teams in England and this summer they are working towards good signings to break that [trophy-less] streak."
Despite not being a permanent fixture under former coach Jose Mourinho, Higuain still scored 16 goals in 28 Liga appearances as Madrid finished second last term.
The Argentine's goal record is also understood to have made him a target of Juventus.