The England international's current deal at The Hawthorns expires at the end of the season.
West Ham and Southampton are amongst the clubs believed to be interested in signing Johnstone in the summer.
But Albion have opened talks with the 28-year-old in a bid to keep their prize asset at the club.
And Ismael says those talks will intensity at the turn of the year.
Asked if there had been any update on Johnstone's contract talks, Ismael said: "There’s nothing new. It’s a patient game now, like a chess game. He wants to see.
"For sure, we want to extend. At the minute, I’m not worried, it’s a normal process.
"From January we will start to go more intensively in the discussion but at the moment we’re all focused on the pitch, on the performances."
Asked if Albion will have more chance of keeping Johnstone if they win promotion to the Premier League, Ismael continued: "We know what we have to do to give ourselves a good position for the negotiation with him.
"When your contract is up you have the choice and I completely understand the situation.
"My job is to make sure that West Brom is in a great position and that West Brom is the right club for him.
"I have spoken frequently with him, not only about the contract situation but also about the national team, about games, build-up, and how we can improve."
Despite currently playing his football in the Championship, Johnstone started for England against Andorra last month.
He is also expected to be in the Three Lion's squad for the upcoming World Cup qualifiers against Albania and San Marino.
And Ismael is delighted that playing in English football's second tier hasn't held back his goalkeeper.
"I’m very delighted about that – that it doesn’t count if you’re a Premier League or Championship player, it’s all about the quality," the boss added.
"Sam has the quality to play. I think it’s clear he’s part of the plan for the national team.
"That is a great feeling for us and for him.
"It’s the reward for his performance at that level."