Sabonis is a very nice player, but he doesn't provide the defensive profile that we need for a big. He's a power forward.
The C's neutralized him pretty well in the playoffs. He's a very clever offensive player who makes a lot happen, but it seems if you turn up the effort, interrupt his offensive sets and focus on putting a body on him, he doesn't have a counter for that.
The reason Brown is such a valuable guy is that he has another gear. A lot of other players shrink in the playoffs, or struggle against superstars. He does not. This is a superstar league, and you need guys confident, skilled and athletic enough to not get overwhelmed. He's one of those guys, even if he doesn't become a superstar.