The defender was then left out of Manchester City’s starting line-up for their midweek win against Watford.
“He’s someone who needs to learn many aspects of his play,” Martinez said.
“The value in the market proves to me he’s got the potential to be one of the best centre-halves in Europe but he’s not ready yet – his decision making, he needs to learn certain aspects to make sure he doesn’t make mistakes in areas you don’t get away with in football.”
Martinez also expects another of his former Everton players, midfielder Ross Barkley, to come good.
The 23-year-old has been in and out of the Everton side under new boss Ronald Koeman but Martinez said: “The first time I took my first session at Everton I saw a player that could do things I’ve very rarely seen players do. Ross Barkley has a special natural talent and I’m sure he’ll come through and show his real potential.”
Since taking over the Belgium national team in August, Martinez has guided the Red Devils to four wins from four 2018 World Cup qualifying matches – scoring 21 goals and conceding just one in the process.
He was appointed only three months after getting sacked by Everton, where he had spent three years.
Martinez led Everton to fifth in the Premier League in his first season and Europa League qualification.
Last season the Merseysiders appeared in the semi-finals of both domestic cup competitions – but he lost his job with the Toffees lying mid-table in the Premier League with five home wins all season.
Martinez says he accepts results were not good enough to keep him in a job given the short-term demands of the modern game, but added: “I will always treasure my memories as an Everton manager – reaching the Europa League, getting 72 points, and then last campaign we were so so close to getting to the finals of both domestic cups.
“I still get satisfaction with our long-term project that we had in hand, seeing players that we invested in heavily, giving them big roles like the level of investment in young players like John Stones, Ross Barkley and Romelu Lukaku, that they have taken the football club to a different level.”