Cape Town - Everton defender John Stones will have to learn from his costly errors this season due to the fierce competition within the squad, his manager Roberto Martinez said on Friday.
Stones, 21, was at fault for the opening goal in Everton's 2-1 Premier League loss to Swansea City in January and he conceded a last-minute penalty in a 4-3 defeat by Stoke City in December.
England manager Roy Hodgson also had to defend Stones this week after he lost possession to start a move that ended with the Netherlands scoring a penalty in Tuesday's 2-1 win at Wembley.
"His progression has been remarkable but he's still learning, he's trying to become as good as he can be," Martinez told reporters on Friday ahead of the league trip to Manchester United on Sunday.
"He's well ahead of where he should be. We should be more aware that he shouldn't be under that amount of scrutiny at this stage of his career. It's how you react to mistakes and he has strong character," the Spaniard added.
"He has to learn and has to learn quickly. The competition in our squad will force him to do that."
Everton's leading striker Romelu Lukaku, who has scored 25 goals this season, has fuelled speculation over his future after saying he intends to play in the Champions League.
With nine games left, the Toffees are 13 points behind the final Champions League spot.
"That should be the aspiration for everyone. We want to be in the Champions League and we want to be competing for titles. We want to create a positive momentum," Martinez said.
"We are relying on the performances of the players and we want ambitious players.... Rom is having a fantastic season and that is the only thing that matters."