Out of contract in the summer, Johnstone is expected to make a decision about his future at the end of the season.
But with the transfer window soon opening, the England international is sure to be linked with clubs in January – with Newcastle the latest side reportedly interested in the goalkeeper.
“He (Johnstone) is completely committed to the team and to the club,” Ismael said.
“He’s a very important player for us.
“A great guy, very focused, he plays at a high level.
“He’s very professional and has a desire to be successful.
“Next year is the World Cup so it’s a win-win situation.
“For Sam, it’s about more than running down a contract, it’s more about being able to play at that level for next year, to be a part of that squad who goes to the World Cup.
“Our part is to help him stay on that level and for sure to play at the highest level we can, for that we need to put in the effort.”
Johnstone showed his class last Tuesday when he made an outstanding double save in Albion’s goalless draw at Blackpool.
And Ismael, who played alongside German legend Oliver Kahn at Bayern Munich, is in no doubt the 28-year-old is an outstanding goalkeeper.
“He’s top quality, he’s a international player,” the Frenchman continued.
“He plays at a high level, concentration and what I like about Sam, and this is his top quality, is that if a mistake happens, or if something goes wrong, he’s prepared for the next situation.
“This is top quality. He’s a top player.”
Albion have offered Johnstone fresh terms to remain at The Hawthorns.
But while that offer has been on the table for a while, Ismael says there is no update with everyone at the club instead focused on turning around a run that seen the Baggies fail to win any of their last four games.
“There’s nothing,” Ismael said when asked if there was an update on Johnstone’s contract talks.
“We’re all focused at the minute to get back on the winning way, and we’re all focused and working very hard to get what we want.”