The former PSG defender completed his switch to the Hawthorns yesterday.
Garlick told the WBA website: “We identified central defence as one of the areas within the squad in which we wanted to increase competition this summer.
“Diego is a player we have been aware of for the past couple of years and when we discovered he wouldn’t be rejoining Malaga, we made our move.
“We have very much done our homework on Diego. His CV speaks for itself and we’ve also received glowing reports about him as a person.
“But the good thing is that Diego has also done his homework on us. I understand he’s spoken to the likes of Gus Poyet and Luis Suarez about the club and Steve, who they have both worked with, and, on the back of this, Diego very much wanted to join us.
“He had other attractive offers but we’re delighted he has agreed to join us. His game is very much suited for the Premier League.”