The England goalkeeper is out of contract at the end of the season with reports suggesting clubs in the Premier League could make a move for him next month.
Tottenham, West Ham and Southampton are amongst the teams to have been linked with the 28-year-old.
Albion, though, have opened talks with Johnstone about a new deal to keep him at The Hawthorns.
And Ismael is confident of keeping the keeper until the end of the season at least.
“Yes, we are planning for that,” Ismael said when asked if he expects Johnstone to still be at the club in February.
“We definitely don’t want to lose the best player in the squad.
“For our purpose and what we want to achieve, Sammy is a massive part of our project regardless his contract situation and is a big part of my thoughts.”
Meanwhile, Ismael says the club is busy working behind the scenes to bring in new players next month, including a new striker.
“We are expecting something to happen,” the boss continued.
“It’s clear there is a plan in place. We had a meeting this week and we know exactly what we can do.
“It is a difficult window but now it’s the next step to do the right thing with players and to continue to improve our team, to improve our way to play, maybe give the chance to bring in another fresh player with another mentality.
“But we need to find the right balance of what we can do, what is available on the market and who is available who can play right away to help us.”
While Albion are in the market for a new striker, reports have emerged suggesting Norwich are going to recall Jordan Hugill from his loan at The Hawthorns.
Ismael says he hasn’t spoken to the Canaries about the 29-year-old’s future.
The boss, though, has challenged Hugill to become more clinical in front of goal.
“I think that since he (Hugill) came here he has given us a lot of effort,” the boss added. “I think we can see he has tried to give everything. Some situations have been unlucky for him.
“He definitely should have scored more goals.
“Against Bristol City he could have four goals in the game.
“And in the Blackpool game and in the Nottingham Forest game he had big chances for us.
“As a number nine you need to score goals, this is why you get your money.
“But I think his mentality is right. He works hard in training and sometimes the striker just needs the confidence at the right moment.”