The Blues have lost back-to-back matches and are winless in five, while Millwall have won their last two games and are unbeaten in five.
Birmingham's defeat to Luton Town on Saturday was their seventh in eight matches on home soil, the other game in that sequence ending in a draw.
It is also a defeat that leaves City in the relegation zone heading into the midweek round of fixtures, one point behind 17th-placed Rotherham United after playing two games more.
Dropping into the third tier is now a real possibility for City, then, with the pressure seriously on boss Karanka to turn things around before it is too late.
Wednesday's trip to The Den will be a tough assignment if Millwall's recent form is anything to go by, with the Lions having moved up to 14th thanks to successive wins.
Gary Rowett's side followed up a 4-1 victory over Sheffield Wednesday with a 2-1 win at Reading last time out, leaving them nine points off the playoffs with a game in hand.
That victory over Wednesday in their last home match ended a run of 10 league games without a win at The Den. They scored as many goals against the Owls as they managed in their previous seven at home.
It is not too late for Millwall to gatecrash the top six, though if that is to happen then they need to keep their winning streak going.
However, the last three league meetings between these two sides have ended as a draw, the last two of them without a goal being scored.
Indeed, all six Championship meetings between Millwall's Rowett and Birmingham's Karanka have ended as a draw, with the last three all finishing 0-0.
Millwall Championship form: WDDDWW
Millwall form (all competitions): LDDDWW
Birmingham City Championship form: WLDDLL
Former Queens Park Rangers striker Smith's goal at the Madejski Stadium was just his third of the season, while Wallace and Bennett have six and two respectively.
As for the visitors, Alen Halilovic returned to fitness in time for the Luton match, giving Birmingham a clean bill of health.
Millwall possible starting lineup:
Bialkowski; Cooper, Hutchinson, Evans; Romeo, Williams, Woods, Malone; Wallace, Bennett; Smith
Birmingham City possible starting lineup:
Etheridge; Valery, Dean, Friend, Pedersen; Harper, Sunjic; Leko, Halilovic, Bela; Hogan
We say: Millwall 0-0 Birmingham City
Millwall have won back-to-back games and are unbeaten in five, while Birmingham have lost back-to-back games and are winless in five.
If recent meetings between Rowett and under-fire Karanka tell us anything, though, it is that you should never bet against a draw.