While the Hornets have won two of their first three matches, conceding only one goal, Blues are looking to bounce back from a weekend defeat to Cardiff.
Match preview
Featuring in the Championship's Friday night game, Watford - who have yet to play on a Saturday or Sunday - won a battle of teams relegated from the Premier League by edging out Burnley.
Tom Cleverley's goal at the end of the first half proved enough to secure a 1-0 win, bringing a second home victory for new boss Rob Edwards from two attempts.
Though the Hornets spent much of the second half under pressure - particularly in the final 10 minutes, following Hassane Kamara's dismissal for a last-man foul - they took maximum points to move top of the Championship table before Saturday's fixtures.
They have since been surpassed, but with seven points on the board already - and both Ismaila Sarr and Joao Pedro remaining at the club for now - Watford are expected to stay among the early pace-setters.
Last season's top scorer Emmanuel Dennis may have departed for Nottingham Forest, but Edwards has also overseen the arrival of some impressive signings in the past week, so the visitors arrive in the Midlands confident of at least remaining unbeaten.
After coming through their opening two games of the new season with four points, the prophets of doom were quietened a little at St Andrew's, but Birmingham could not keep the positive momentum going for their weekend trip to the Welsh capital.
John Eustace's side went behind early on against Cardiff City to a goal from Aston Villa loanee Jaden Philogene-Bidace, and they were somewhat fortunate not suffer a greater margin of defeat than the final 1-0 scoreline.
Given their well-documented issues off the pitch, many expect Blues to be locked in another scrap to avoid relegation this year, though they managed a 2-1 win over playoff finalists Huddersfield Town in their only home game so far.
They also fought back from behind to draw 2-2 with Norwich in the EFL Cup, before succumbing to the Canaries on penalties, so there have been signs of hope for Eustace and the club's beleaguered fans.
The former QPR assistant and Kidderminster Harriers boss spent five years of his playing career at Watford, representing Birmingham's next opponents 157 times, but sentiment will have to be in short supply when the Hornets arrive in B9.
Birmingham City Championship form:
Birmingham City form (all competitions):
Watford Championship form:
Watford form (all competitions):
Team News
Watford manager Rob Edwards has a handful of absentees to contend with on Tuesday, as Hassane Kamara must serve a suspension following his red card on Friday, while two players have left the club and others are out injured.
Midfield pair Tom Dele-Bashiru and Imran Louza remain sidelined by long-term knee injuries, and Nigerian forward Samuel Kalu will not return from a hamstring problem until next month. His countryman Emmanuel Dennis has joined Nottingham Forest's revolution for £20m, and centre-back Samir recently signed for Tigres UANL.
Nonetheless, new arrivals Keinan Davis - on loan from Birmingham's arch-rivals Aston Villa - and former Barcelona striker Rey Manaj provide competition for Joao Pedro up front, and Hamza Choudhury should start in midfield after a positive debut against Burnley.
Meanwhile, Birmingham boss John Eustace only made one change to his lineup last time out, with Ryan Woods being replaced by teenager Alfie Chang, and he is likely to retain most of his starting XI again, despite defeat to Cardiff.
The hosts should feature John Ruddy in goal, and a midfield three of Chang, Juninho Bacuna and Jordan James, while captain Troy Deeney is partnered by Scott Hogan up front.
Birmingham City possible starting lineup:
Ruddy; Sanderson, Roberts, Trusty; Colin, James, Chang, Bacuna, Placheta; Hogan, Deeney
Watford possible starting lineup:
Bachmann; Gaspar, Cathcart, Sierralta, Sema; Kayembe, Choudhury; Sarr, Cleverley, Pedro; Davis