The 58-year-old has come under increasing pressure in recent weeks as Spurs have continued to put in underwhelming displays.
The Portuguese boss appears to be losing popularity amongst large sections of the Tottenham support, and Castles believes sacking him would now be an easy decision for Daniel Levy.
He said: (23:08) “It is now, I think, very easy for Daniel Levy to dismiss a renewal if he decides to go down that route. There’s, you know, huge criticism of just about everything he does.
“And a disgruntled supporter base who want to see a change in manager and also, I think, want to see a change in players, which is something Mourinho wants but is aware that is going to be difficult to do in the transfer window coming because Tottenham are not, at present, prepared to put a big budget into reinforcements.
“Although they accept his argument that the defence at the very least needs to be upgraded in the summer if they are to do what Mourinho was brought into the club to do, which is compete at the top end of the table.”
TIF Thoughts on Mourinho…
Mourinho was brought to the Tottenham Hotspur Stadium to end the club’s lengthy trophy drought, but early exits in the FA Cup and Europa League have left the Carabao Cup as their only chance to secure silverware this season.
Several sources have claimed that Levy will axe the ex-Chelsea man if Spurs miss out on a top-four finish, and it’s hard to imagine many Tottenham fans protesting at that decision.
The north London outfit have the chance to win their first major trophy since 2008 when they take on Manchester City at Wembley on April 25, and the result could have a significant impact on Mourinho’s future.