Keeping Mangala for this season would be a great decision in my opinion. There will be certain matches and particular situations within matches that he will be useful. He also seems like a good professional and would accept his role as fourth choice. If a CB that a) improves on who we have already or b) has the potential to be world class isn't available this summer then it is wise to wait until next year (or January if needs be) before spending a large amount on a player this summer who is neither of the above. If we can bring in another attacking player (or two) before this deadline then next summer we can (hopefully) buy both a world class centre back and one or two holding/defensive midfielders/pivots (Weigl for me).