Ajayi limped off at Wigan on Tuesday night and Bruce feared the worst on what initially sounded like a worrying knee and ankle problem in an innocuous incident.
But scans have revealed no issue with the knee and ligament damage after a bad twist of the ankle, which was actually sustained during Josh Magennis' opener for the Latics, in which he shrugged off Ajayi.
Bruce has just under 12 hours of the transfer window remaining to complete his summer business and revealed Albion 'had something up their sleeve defensively' in case of any further issues at the back.
"He's got an ankle injury. I don't want to put a timescale on it," Bruce said this morning.
"He'll certainly miss a few weeks. That's certain. We're unfortunate, what can you do? He's twisted his ankle badly. These things happen. It's him now and Dike.
"The games are coming thick and fast and there lies another one – we've unfortunately lost a big player.
"His knee is okay. Whether it was nerve damage from the ankle, we've scanned them both. He's going to be a few weeks. It's ligament damage.
"He's badly twisted his ankle, we've got to be patient but it'll be a few weeks. He's a tough lad, because he did the injury in the incident before their goal. He played on for 20 minutes, that was a strange one."
Callum Robinson is set to leave The Hawthorns for Cardiff City on deadline day after the Baggies added League Two forward Brandon Thomas-Asante to their ranks from Salford City yesterday.
Asked if Ajayi's setback has a say on recruitment plans today, Bruce said: "Slightly, but we always had something up our sleeve anyway because when you look at us defensively we're a bit short. We'll look to do something there."