Mourinho has spent £300million since being appointed Manchester United manager in May 2016, with Paul Pogba, Romelu Lukaku and Henrikh Mkhitaryan all arriving for big fees.
However, the Red Devils still need more if they are to stop noisy neighbours Manchester City winning the Premier League title with ease.
Mourinho has been told he must sell before he buys, with the club keen to recoup some money for the likes of Luke Shaw, Daley Blind and Matteo Darmian.
And the Sun say the Manchester United boss is ready to wage war against the board as he feels more funds should be made available to him.
The 54-year-old is said to be concerned that the club will not back him in the January transfer window.
Mourinho seemingly still wants reinforcements amid injury concerns over Paul Pogba and Zlatan Ibrahimovic.
Both players are nearing returns but Mourinho does not want to rush them back out of fear that, in doing so, it could hamper their contributions on the pitch.
That is why he wants to strengthen in January but it appears he may not be allowed too.
GETTY
Man Utd news: Jose Mourinho is ready to challenge the Old Trafford board over transfer policy
GETTY
Man Utd news: Jose Mourinho remains concerned about Zlatan Ibrahimovic and Paul Pogba
The Sun point out that transfer angst comes at a time where Mourinho's future is uncertain.
Ligue 1 giants Paris Saint-Germain are supposedly readying an attempt to prise him away from Old Trafford at the end of the season.
PSG want Mourinho to ensure Neymar stays - with the Brazil international reportedly unhappy under current boss Unai Emery.
While Mourinho is happy at United for now, war with the board could push him further towards the Parc des Princes.
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