The Bees sold the Dutch keeper to Bayer Leverkusen on Tuesday.
Giles told the Brentford website: “Mark has proven to be a seamless replacement for David Raya, who set an extremely high standard during his years with us.
“We were expecting Mark to be our goalkeeper for many more years, however when Bayer Leverkusen made it known that they wanted to do this transfer, and Mark made it clear that he was interested in moving closer to home and with a possibility of Champions League football, we turned our attention towards how all parties could make this happen.
“Mark has done a great job for us and leaves with our thanks and support.”
A great keeper on and off pitch
Bees goalkeeper coach Manu Sotelo added: “Mark was a great no.1 for us on and off the pitch.
“Despite a tough beginning, he ended up being a really important player. Offensively, he helped us to play from the back on another level, but not just playing short as he got three assists in two seasons.
“Defensively, he became a really solid goalkeeper after an uncomfortable transition after changing countries. The Premier league is ruthless, but he became one of the best goalkeepers in the league.
“He was relentless in training and leading our young goalkeepers. His impact here was massive and we wish him all the best for the future.”