Speaking ahead of the weekend’s match against Boavista, Conceicao hit out at the 36-year-old for not treating his sessions with the expected urgency.
The former Nantes boss has established himself as a strict disciplinarian and has revealed that the pair have held talks to explain why Jose Sa has instead been trusted with the responsibility of keeping goal for the Portuguese giants.
“He’s trained below what I demand,” he said. “The players know what my rules are, how I lead and that I demand that everyone is always at 100 per cent in training.
“I have to be consistent in the locker room, regardless of whether the player’s called Joaquim, Manuel or Antonio. If I make exceptions, I lose the whole squad. So I won’t do that.
“I can’t close my eyes on two weeks of training that did not reach my expectations and Iker cannot understand this. He didn’t train with the energy that I demand.”
Casillas is largely remembered for his spell at Real Madrid, where he spent 25 years overall and won every major club title open to him. He also picked up a raft of honours at international level, winning the World Cup and two European Championship titles with Spain among his record 167 caps.
Now in the twilight of his career, he faces the unusual prospect of a spell on the bench, having previously been placed there notably by Jose Mourinho when the Manchester United manager was in charge at the Bernabeu.
He has played 10 times for Porto so far this season, helping the club to first place in the Portuguese top flight, though they are struggling to make progress in the Champions League, having lost two of their opening three fixtures.
Despite keeping seven clean sheets and conceding only six times in his 10 appearances, it seems he faces a major battle to regain his berth as a starter at Estadio do Dragao.
By contrast, Sa has played four times and has conceded as many goals, keeping only two clean sheets.