The Gunners midfielders had a warm message for his compatriot.
According to the Daily Mirror among others, Arsenal wanted to move for the goalkeeper as recently as last month, eventually signing Chelsea shot stopper Petr Cech.
The Spaniard has now ended up joining Porto and fellow countryman Cazorla has congratulated him on his move.
Mucha suerte a @CasillasWorld en su nueva etapa,el mejor portero del mundo sin duda y una gran persona mucha suerte amigo— Santi Cazorla (@19SCazorla) July 12, 2015
The message translates: 'Good luck to @CasillasWorld in his new phase, the best goalkeeper in the world without a doubt and a great person good luck friend.'
The chain begins?
Arsenal and Porto may have their new goalkeepers but that leaves Real Madrid still needing one. Will they move for Manchester United's David de Gea? And will that prompt United to raid Tottenham Hotspur or anyone else?
Casillas's move may indeed have started a goalkeeping chain this summer, unless, of course, Real Madrid end up settling for someone other than De Gea.
Casillas will no doubt hope for more titles at Porto, as well as potential Champions League progression. Cazorla's claim that the 34-year-old is still the best goalkeeper in the world, though, is more the kind words of a friend rather than fact. In terms of history, yes. On current form, not a chance.
Meanwhile, Mesut Ozil also paid tribute: