The Blues boss was speaking to the media ahead of the clash at the Hawthorns at the weekend.
Striker Romelu Lukaku trained for the first time since suffering a heel injury against Espanyol two weeks ago.
The Dutchman also added that right back Seamus Coleman is set to miss another fortnight’s worth of football after he missed last weekend’s draw with Tottenham.
Koeman said: “Romelu did his first training session this morning without any problem. I need to make a decision tomorrow if he is part of the squad for Saturday.
“[Ashley] started pre-season later and he has had five days [training at Finch Farm] and is closer to being part of the team. Maybe he will be in the squad. We need to make the decision tomorrow.
“He (Bolasie) will be part of the squad but if he starts or is on the bench, we will decide tomorrow.
“Seamus is not ready. He’ll be out for another two weeks at least.”