The Montenegro goalkeeper sadly passed away at the age of 26 on Saturday.
He was in Budva at the time, with everyone associated with the sport paying their respects in the past few days.
Speaking on Millwall's official website, Harris said: "First thoughts have to go with Mati's parents and his brothers. They had a wonderful son and a wonderful brother who was a really caring, good man.
“Obviously getting the call at 7 o'clock in the morning first thing on the weekend, you know it's not going to be positive news. But you could never imagine the devastation in the moment that you find out that such a good man at such a tender age has lost his life.
"Words find it hard to explain how you feel. It makes it very emotional to me because I have a 22-year-old son myself, Mati was only a few years older. Any human being, any Millwall fan or anybody in life can relate to something like that. (It's) devastating news for the family, for our football club."