The Red Devils have lost their last two Premier League away matches, at Tottenham and Newcastle, and Mourinho is preparing to reshuffle his defence for the upcoming FA Cup tie against Huddersfield.
Chris Smalling and Phil Jones are the players that will make way, with Mourinho expected to give Victor Lindelof and Marcos Rojo the opportunity to impress.
However, in the longer term Mourinho feels he needs to heavily invest in his defence when the market reopens and The Mirror say he has already identified the centre-backs he wants.
The first is Real Madrid centre-back Raphael Varane, who Mourinho brought to the Bernabeu when he was in charge.
United are hoping to profit from the growing turmoil at Real, with the Spanish champions offering a meek defence of their title – they have fallen 17 points behind La Liga leaders Barcelona.
The other centre-back interesting Mourinho is Leicester City’s Harry Maguire, who has impressed enormously in the last two seasons and would cost around £50m as a result.
But Mourinho has told United chiefs that the club must start spending big on defenders to match their Premier League rivals.
Manchester City spend £57m – a club record – on Aymeric Laporte, while Liverpool broke the transfer record for a defender by splashing out £75m on Virgil van Dijk, and Mourinho wants United to follow suit.