The Premier League champions are expected to dip into the transfer market this winter following a woeful start to the season.
The Red Devils currently occupy a position in the bottom half of the standings after six games, which has led to criticism for new boss David Moyes.
Following a difficult summer on the transfer front – where Marouane Fellaini was the only notable arrival at Old Trafford – the Scot is already lining up targets, with Mangala near the top of his list.
Moyes is keen to bring in a young centre-back as he looks to plan for life without the aging Rio Ferdinand.
Mangala, 22, is thought to be his player of choice with a January offer rumoured to be on the cards.
However, Porto are desperate to keep their enforcer and are in no rush to sell.
Having sold James Rodriguez and Joao Moutinho to Monaco for a combined £70m of late, the Portuguese giants are far from being strapped for cash.
But, they will allow the Frenchman to leave if United pay £25m, with the profit on the man they signed for a minimal fee from Standard Liege too high to resist.
Fierce rivals Manchester City are also said to be keen.
Should Manchester United splash the cash for Eliaquim Mangala?
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