The defender limped off in the first half of tonight's 1-1 draw at Wigan in the Championship having made an error for the Latics' opener at the DW Stadium.
Ajayi has impressed overall for the Baggies this term but Bruce is now prepared for the worst on the defender's prognosis after the boss revealed he hurt his knee and ankle in an innocuous incident with no Wigan player close to him.
A possible extended absence comes as another hammerblow to Albion, who are already working with a thin squad they are attempting to bolster before Thursday evening's transfer deadline.
Kyle Bartley was recalled to the Albion backline to play alongside Ajayi in Wigan this evening as captain Dara O'Shea was shifted out to right-back, but that only lasted 20 minutes before Ajayi pulled up. The defender has been sent for a scan and there are fears the setback could be severe.
"Not great. We'll not know until the scan but it doesn't look great, I have to say," Bruce said when asked about Ajayi's injury.
"He's done his knee and his ankle, they are not looking good great.
"I'm not anticipating good news on that, that's for sure."
Asked if the latest fitness blow could force Bruce to look for cover at centre-half before Thursday's deadline, Bruce added: "You can't cover for every eventuality but - what can you do? We've lost two big players, which is cruel."