What do we know about Sakari Mattila? Not a lot actually. The Tippeligaen isn't a league many of us watch a lot of. We know that he's big (6'2"). We know that he's not old (only 26). We know that he's primarily a defensive midfielder but can play center back in a pinch. Is he a savior? No he looks to be more of a depth player. For all the players Fulham have in the midfield, they don't have a lot of straight defensive midfielders on the roster.
The big question though, how good is this guy? The Tippeligaen is a few rungs below the Championship and he wasn't exactly dominating there. It's always possible Fulham know something we don't. It's possible there is some statistical/moneyball reason for this signing. But even if it doesn't work out at all, what have you lost? I can't imagine his fee was more than a few hundred thousand, and he's unlikely to be on high wages. Doesn't hurt to take a flyer on a few guys like that.