The West Ham United midfielder recently switched from Ireland to England.
Keane said: "I've got some sympathy for him. It was not an easy decision for me at the time and I can imagine it's harder for him.
"You've got to go with your gut feeling and where you feel like you belong.
"You have to go with where you think you could end up in your career.
"Declan has made his decision now and we are happy that he is here. Hopefully people now let him display his talents on the football pitch."