You must also bring his age into consideration here, he is well over 30, I do not know if united are thinking here trying to get mandzukic in the deal.
Manchester United are trying to buy Mandzukiz as a short term fix, because they know they’ll be ripped off if they try to buy a new striker in middle of the season. At his age I don’t expect him to start each and every match, but he’ll play from the bench and will only start if Rashford or Martial are injured. I’m not sure whether this decision will help Manchester United or no, but personally I would prefer the club buying Zlatan over Mandzukiz as Zlatan is guaranteed to score 10 - 15 goals in the remaining games.