Pereira sat out all of the Baggies pre-season fixtures in order to avoid an injury which might scupper his chances of a move to the Premier League.
That arrangement is now set to extend to competitive fixtures with Albion due to start their campaign at the Vitality Stadium on Friday.
Ismael said: “I have spoken with Matheus for a few weeks. He was clear he wants to leave. He is not committed to the Championship and not committed to us.
“I accept this decision but I have to take my decision and now we need to solve the situation.”
Albion are prepared to sell both Pereira and goalkeeper Sam Johnstone in order to boost their transfer budget but no deal appears close for the Brazilian playmaker.
Ismael continued: “At the minute we are talking but there is nothing clear. At the minute I am just focused on the guys who are ready to play for West Brom, ready to embrace the fight in the Championship.
“We need to have players who are motivated to work for each other. I don’t need the best player or the most expensive player. I need the right player with the right mentality.”
David Button is set to start in goal for the Baggies at Bournemouth, with Johnstone still getting up to speed after returning to training last week.
“He needs some time to clear his situation,” said Ismael when asked about Johnstone. “We will have some talk in the next few days but at the minute it is important to focus on the players who are available.”
Matt Phillips is available following a period of isolation which saw him miss Saturday’s 4-0 win at Blues.
Midfielder Quevin Castro will be assessed after sitting out the final pre-season friendly having dislocated his shoulder in training on Friday. Goalkeeper Alex Palmer missed the match due to a concussion but should be ready to back up Button at Bournemouth.
Albion are, meanwhile, expected to complete the signing of free agent winger Adam Reach early this week.