He had the names of Harry Maguire, Yerry Mina, Diego Godin and Jerome Boateng on his transfer wish-list – but moves for them all fell through.
Instead, United had to make do with Diogo Dalot – a teenage sensation with bags of potential from Porto, but far from the finished article Mourinho was looking for.
Mourinho is now said to be keen on resuming his centre-back hunt while also signing a midfielder.
“We all know he wanted a centre-back in to strengthen the team and was barred from having that centre-back in,” Castles told the Transfer Window Podcast.
“He is still trying to get a centre-back in. He’s also looking in midfield.”
United are believed to have changed their transfer policy to only sign players with a possible sell-on value.
While bringing in players appears to be the priority, Paul Pogba could be on his way out with Mourinho said to be keen to show the French midfielder – an £89million signing two years ago – the door.
“There’s clearly the potential there, a strategy there, to say to the club ‘if you’re not going to fund properly a centre-back, allow me to cash in on still the most valuable player in the squad’,” Castles said.
“’Then I can reinvest that money in the positions I think need to be strengthened for us to be properly competitive in the Premier League and to continue our Champions League campaign and to have a chance to go deep into that tournament’.”
Off the pitch the United boss has been at war with Pogba, who he is said to have branded a ‘virus’ in a tense dressing room exchange in front of his United team-mates.
Part of Mourinho’s problems with the Frenchman, Castles says, is that he slows play too much while he also wants to show he can stand up to a big character.
Castles added: “He wants one or two touch football, he wants the ball moved quickly and Pogba is often the guy holding him up.
“There’s also an element in terms of a certain power to show he’s prepared to stand up to a player who is hugely commercially important to Manchester United, and is aware of his commercial importance.
“He’s letting the rest of the dressing room know that what he sees from Pogba is unacceptable and he’s prepared to take action against him.
“Then there’s the key element that we need to talk about which is a challenge to Ed Woodward. Who is more important to you – Paul Pogba or Jose Mourinho?
“And are you prepared to consider the possibility of selling Pogba, cashing in and finding another club to move him to and reinvesting that money in the team because we all know that he wasn’t given enough money in the summer window.”