The Red Devils sit sixth in the Premier League having won just one of their last six games in the local league.
Most recently the former league champions have suffered heavy home defeats in the hands of arch rivals Liverpool and Manchester City.
United brought in Cristiano Ronaldo, Jadon Sancho, and Raphael Varane from Juventus, Borussia Dortmund and Real Madrid respectively for more than £130million during the summer.
Their Champions League hopes still lie in the balance being faced with the task of beating Villarreal and Young Boys to guarantee their progression.
Castles was quoted by the Mirror, UK, as saying that senior players in the squad now feel it’s time for the club to bring in a new manager.
According to Castles, the senior players are dismayed that United won’t achieve what they are capable of doing this season.
One of such senior players, according to the report, is Jadon Sancho, following claims that Solskjaer was preparing the England attacker for defensive duties at Manchester United after the international break.
Since Alex Ferguson left the club in 2013, the red devils have struggled to relive the past days of glory when they won everything in the round leather game. They have brought in world class coaches like Jose Mourinho who was also booted out unceremoniously.