Albion were beaten 2-1 by Bristol City at Ashton Gate in the Sky Bet Championship on Tuesday evening.
Following a goalless first-half, the Robins edged ahead through Nahki Wells’ long-range strike.
Tony Mowbray’s men responded courtesy of Alex Mowatt’s tap-in, though City stole victory in added time via Haydon Roberts’ effort.
The Baggies are back in action this Saturday, hosting Watford.
The head coach made four changes to his XI for the trip south, switching in Josh Griffiths, Mason Holgate, John Swift and Isaac Price.
Albion created the two best chances during a nervy opening half-hour, with Price sending a volley wide and Kyle Bartley seeing a close-range header comfortably saved.
Before half-time, Griffiths was forced into making an outstanding double stop to deny City’s Wells - even if the assistant referee did later put his flag up for offside.
And the Baggies goalkeeper superbly kept out Ross McCrorie, too, ensuring proceedings were level come the interval.
Further opportunities were exchanged straight after the restart - as Griffiths kept out Wells’ finish while Armstrong had a poke held from inside the area.
But City opened the scoring on 56 minutes, Wells finding the bottom corner and handing them the lead.
However, it didn’t last long. Price’s long ball over the top caused a huge defensive mix-up, and Mowatt made no mistake to tap in and level the scoreline.
Momentum immediately swung and the Baggies then almost went in-front, substitutes Tom Fellows and Ousmane Diakite combining for the latter to see his header pushed away.
Daryl Dike - also brought on - almost won the match, agonisingly watching Max O’Leary keep out his late attempt.
Instead, Bristol City stole it - taking advantage of Jayson Molumby’s red card and scoring during stoppage time.
Albion: Griffiths; Furlong, Holgate, Bartley, Styles; Molumby, Mowatt (Diakite 65); Price (Fellows 65), Swift (Bany 65), Grant (Johnston 80); Armstrong (Dike 65).
Substitutes: Wildsmith, Frabotta, Diangana, Lankshear.