I don't think that Premier League proven is really that big an issue when you have won the Champions League where you were man of the match, the domestic league in 2 different leagues with 2 different styles of play, lead your team to the quarter finals of a World Cup, being voted the second best player at the World Cup and the joint top goalscorer.
You're kind of football proven by then.
He's not an unknown player, we all know what he's capable of. No player is PL proven before they come here, but we still keep buying them because they all know how to play football.