With that argument, every player makes bone-headed passes. There is no player who doesn't make the occasional mistake. For as much as Fowler handles the puck, and he handles it a lot, the amount of time he makes a mistake with the puck is remarkably low. You're going to see mistakes from any player, and the more they handle the puck, the more likely you are to see a mistake.
I'm not going to deny he can be weak on the puck. He isn't an especially strong player, and he isn't a particularly aggressive one either(see: Vatanen). His skating, and ability to separate himself from opposing players more than makes up for it. Even being a bit weak on the puck, he has a ridiculous ability to escape tight spots. If you give him an inch, he'll turn it into 6 feet. I'd take that over being stronger on the puck, because it's something he can do consistently, and size isn't a factor. A player his size, no matter how strong on the puck he is, will lose battles. But his skating and ability to separate himself? That works against any player.
As for the crease, again, not a big guy. Not a strong player. Vatanen is feisty as hell, and he can't clear the crease if his life depended on it. I'd actually prefer that he not get locked up with players, because if there is a loose puck he's the guy who can get to it. He's better off trying to take away their stick, but keeping himself available.
But the bottom line is that Mantha isn't going to get you a Cam Fowler. Fowler, at 24, is the type of player(albeit at a different position) that you'd hope Mantha becomes. An equivalent forward. That puts all the risk on Anaheim, because even if Mantha turns out great, Detroit would be guaranteed to get a great player in the deal. Anaheim would be the one taking the chance. So, yes. You'd be adding to Mantha. Probably a fair bit. That isn't an outrageous statement. An established high end player is worth more than high end potential. It's worth a lot more. That doesn't mean Mantha sucks, or is some toss in. No one is saying that, just so we're clear, but at 22 Mantha isn't exactly a sure thing.