The Bluebirds had been on a great run prior to losing 2-0 to their bitter rivals, while Birmingham also saw an upturn in form come to an end last time out with defeat to Watford.
Saturday's defeat to Swansea was Cardiff's fourth loss on home soil this term, which is already one more than they lost in each of their previous two seasons at this level.
That return of 12 points from 12 had lifted the Bluebirds right back into promotion contention, but they are now five points off the top six heading into the midweek fixtures.
With two members of the top six to come over the fortnight - a trip to Norwich City followed by the visit of Brentford - Cardiff need to quickly pick themselves up.
This may well be just the fixture for the Bluebirds to do exactly that as they have won five of their last seven home league matches against Birmingham, drawing the other two.
In fact, the Blues have won only two of their last 15 league meetings with Cardiff, while boss Aitor Karanka has lost on his last two visits to the Welsh capital.
Birmingham suffered a 1-0 loss to Watford last time out through Troy Deeney's late penalty in a match devoid of any real action until that point.
Karanka conceded after the match that his side are making too many sloppy errors, such as the one to give away the spot kick, with his side now down in 16th place.
The Blues have defeated Bristol City and Reading in their last two away games, though, and they are unbeaten in their last six matches away from St Andrew's.
Cardiff City Championship form: LWWWWL
Birmingham City Championship form: DDLWWL
Neil Harris had understandably been reluctant to rotate his side on the back of a positive run heading into the Swansea game, but that will surely now change.
Cardiff will be forced into making at least one alteration due to Ralls's suspension, with Marlon Pack the favourite to come into central midfield.
Jeremie Bela has also been ruled out through injury, but Karanka otherwise has a fully-fit squad to choose from.
Cardiff City possible starting lineup:
Smithies; Bacuna, Morrison, Nelson, Bennett; Wilson, Vaulks, Pack, Ojo; Glatzel, Harris
Birmingham City possible starting lineup:
Etheridge; Colin, Roberts, Dean, Clarke-Salter; Kieftenbeld, Sunjic; Sanchez, Toral, Leko; Hogan
We say: Cardiff City 1-1 Birmingham City
Cardiff had an off-day against Swansea and will know the importance of quickly hitting back if they are to compete their playoff aspirations alive.
Birmingham may also have tasted defeat last time out, but they have a good away record and will be confident of taking a point back to the West Midlands.