The defender has not featured since the end of last month, where he sustained the ligament damage early on in the 1-1 draw at Wigan.
He went under the knife to rectify the problem and boss Steve Bruce has now confirmed the defender will spend up to two months out of action.
That could rule Ajayi out until after the break for the World Cup. Albion’s final fixture before the month-long pause is Saturday, November 12 at home to Stoke.
Bruce said: “We knew it was a huge amount of games in a small period of time. We’ve had one or two injuries which have unfortunately for us been serious in that time.
“Semi has had an operation on his ankle. We felt it was the best forward, so he’ll be another six to eight weeks or so.
“(Daryl) Dike you know about. The rest have bumps and bruises but everybody has come through.”
It was initially feared Ajayi had also sustained significant damage to his knee during the incident at the DW Stadium, though scans showed all clear in that regard. Albion have still, however, moved to add defensive cover since Ajayi’s setback with two free agent additions who are primarily full-backs but can switch to the middle of defence.
Martin Kelly penned a two-year deal on deadline day following his release from Crystal Palace and Erik Pieters yesterday finalised a one-year contract at The Hawthorns after exiting Burnley.
Pieters said after putting pen to paper yesterday: “I now want to prove myself and show what I still have in me. It’s down to me to prove that I’m worthy of being signed here and that I still have what it takes to play at this level of football. I’m confident I can do that.
“When my body is telling me that it can’t handle it anymore then that’s when I’ll have to think about what the future holds, but I still feel great.”
Bruce also confirmed that striker Daryl Dike (thigh) and defender Kelly will not be ready to feature until after the upcoming international break, which begins after this weekend. Dike is midway through a muscle tear rehabilitation and Kelly is building up his fitness.