Only goal difference keeps Latics out of the drop zone at present, while their financially-stricken north-west neighbours will have relegation to npower League Two confirmed by defeat.
"You could call it anything - a derby, a cup final," said boss Johnson. "The biggest game of the season? Of course it is.
"We've got to make sure that we're on song because if you're not you can get done in those games.
"We shouldn't be down there but you can't keep saying that because we are and it's proven over 41 games that there's something wrong. We need to make sure we address it and come through it very quickly."
Johnson once again pointed to fitness issues within his newly-inherited squad as they fell away to a 2-0 defeat at Preston in midweek having enjoyed much the better of the first hour.
"There's nothing that can be done about that now," he added. "How can you improve fitness with games Saturday-Tuesday, Saturday-Tuesday? Probably from the games, fitness gets maybe an extra one or two per cent every game but you always run the risk of over-cooking it."