The player is still young though.
Mandzukic to Barcelona would be an unwise move for the player as he'll see limited playing time, and I know he'll be briefed about that should the deal be on the verge of pulling through.
Mandzukic is definitely coming to be a backup and he better think twice if that's what he needs at this stage of his career, he'll eventually become surplus to requirement at some point and be moved on, just as Barca did you Boateng last season, a club with a lesser pedegree would fit Mario more.
Mandzukic is a rare breed of player. He is a fighter in every possible meaning of that word. He relishes a battle for first team positions while most other players start to be unhappy and wait for the position to open up for them. He has been in second or third attacking option at the beginning of every season in Juve and Bayern and somehow manage to battle his position to the top of pecking order while getting most of the supporters on his side as well.
He is not getting any younger so easier league and guaranteed starting spot would probably be safer option but I somehow doubt he will choose the easy way.