After impressing in the Premier League last season, Johnstone had been expected to leave The Hawthorns this summer.
West Ham tabled a derisory offer of £6million to land the England international whose current contract expires next summer.
But with few top flight clubs in the market for a goalkeeper, a move for Johnstone hasn't materialised.
Part of the England squad that got to the final of the European Championship, Johnstone returned to training early this summer in a bid to impress new boss Ismael and show his commitment to the club.
And while the boss says anything can happen in a transfer window, he now isn't expecting Johnstone to leave the club this month.
"Yes, we are working with that expectation," Ismael said when asked if he expects Ismael will stay.
"After Sam came back to training, I was my job was to analyse the situation, the new situation.
"I looked at the attitude of the player, the mentality of the player, does he want to be part of the process?
"Or is the thought more I want to leave, I stay focused on my transfer?
"Sam had the right attitude, especially at Bournemouth.
"He was not involved but his attitude in training before the game, after the game was positive and very professional. It’s what you want to see as a manager.
"Now the time for me is to take that decision, because the player is ready, he wants to play.
"And when the player has the right mentality we want to play with the better team.
"I think Sam showed over the last three years – last season was a brilliant season from him – that he is number one.
"I think it’s a logical step that Sam comes back in between the sticks.
"Now my expectation is that his focus stays on West Brom.
"Anything can happen. But we started to work on his contract and we are in talks now to try and get an extension on his contract.
"I think that is a good sign. Things can still go in all directions, he could leave but he could stay – how I think about is Sam he is our player and we will need him."
Asked if Johnstone will start tomorrow, Ismael said: "Sam Johnstone will play from the beginning.
"His attitude has been great.
"He came back early and it was the feeling now that we need to have clarity in that position.
"We have taken the decision now that Sam will start."
One player who won't be involved tomorrow is Robert Snodgrass who is back in training following a back injury. And Ismael revealed he has earmarked the League Cup game with Arsenal for the Scotland international to make his return.
"We need to have the build-up with him," Ismael added.
"Next week he’ll play a bit longer with the 23s, now we have a League Cup game against Arsenal, it’s a chance for us to give him more minutes and more time in that game.
"We have a plan, the next two or three weeks with Snods, we need to make sure every time he reaches the step we want. At the minute, he’s good, he looks really good, and we work on the next step next week."