39-year-old Johnson had been in the role since February 2016 and was the longest-serving manager in the Championship but was relieved of his duties following a nine-game winless run, including a fourth successive defeat against Cardiff City on Saturday.
Assistant Head Coach Dean Holden has been handed the role on an interim basis as the Robins look to appoint a permanent successor, with his first game in charge seeing the Tigers provide the opposition in BS3.
“We can expect nothing but a tough game tonight,” said Magennis. “Bristol City are going to want to bounce back after a frustrating run of results and Lee Johnson’s departure.
“That was sad to see because you don’t want that to happen to anybody in this game.
“But, the show must go on and they’ll no doubt be out to impress the interim Head Coach or any possible future candidates for the role. One thing that’s for sure is that they’ll be looking for a reaction.
“We’ve just have to make sure we’re prepared for that and make sure we’re focused on what we have to do to get the result we all want and need.
“It’s another quick turnaround after West Brom on Sunday, but we’re all looking forward to the challenge and more than ready to go into battle once again tonight.”