The Robins took action after a winless run of nine matches that leaves them nine points off the playoffs, while Hull are also in need of a win to climb clear of the dropzone.
Saturday's late 1-0 defeat to Severnside rivals Cardiff City proved to be the final straw for Bristol City's board as they parted company with the division's longest-serving boss.
That loss was City's fourth in a row since returning to action last month and extended a winless run that spans all the way back to early February.
That most recent victory - 3-2 against Derby County - moved the Robins to within three points of the automatic promotion spots, with Johnson's reputation at a high.
However, they have collected two points from a possible 27 since then and are now nine points adrift of sixth-placed Cardiff with only 15 points left to play for.
Assistant boss Dean Holden will take charge of this clash with Hull and will be eager to steer Bristol City clear of a fifth straight loss, something last achieved in January 2017.
While the hosts' season is as good as over following the defeat to Cardiff, Hull are battling for every last point on offer in their bid to dodge the drop into League One.
The Tigers beat Middlesbrough 2-1 last week, but they followed that up with a 4-2 loss at West Bromwich Albion on Sunday and are one point and one place above the dotted line.
The win over Boro was Hull's only three-point haul in 15 attempts since New Year's Day - a run that has seen them go from promotion challengers to relegation candidates.
Grant McCann's side still have fellow strugglers Wigan Athletic and Luton Town to face in their final five fixtures, but they will be targeting at least a point on Wednesday.
However, they have won only one of their last 22 away league matches against Bristol City, losing 15 of those, and are winless in five against the Robins overall.
Bristol City's Championship form: DDLLLL
Hull City's Championship form: LLLDWL
Striker Nahki Wells has scored 16 goals in the Championship this season, one short of his best return in the Championship when at Huddersfield Town in 2015-16.
As for Hull, Liverpool loanee Herbie Kane picked up an injury shortly after scoring against Middlesbrough last week and is a confirmed absentee for this match.
Bristol City possible starting lineup:
Bentley; Hunt, Kalas, Baker, Dasilva; Paterson, Smith, Massengo, O'Dowda; Diedhiou, Wells
Hull City possible starting lineup:
Long; Pennington, Wijs, Burke, Lopes; Wilks, Batty, Stewart, Scott; Toral; Magennis
We say: Bristol City 1-0 Hull City
Bristol City are embarking on a new era after parting company with long-serving boss Johnson. We are tipping them to pick up a much-needed victory on Wednesday against a Hull side at serious risk of dropping down a division.