After six successive wins in October, the Blues lost 2-0 at Newcastle on Saturday to extend Mourinho's winless run at St James' Park.
Chelsea return to home Champions League action for the first time since the loss to Basel which ended a near-10-year, 29-match unbeaten run in the group phase of Europe's elite competition.
The Blues responded by winning at Steaua Bucharest and Schalke and another victory over the Germans would enhance claims on a place in the knockout stages.
"We are in a stable position to qualify in the Champions League but if we lose again we are in a difficult position," Mourinho said on chelseafc.com.
"So Wednesday is an important game for us."
Chelsea have won all five of their Barclays Premier League home fixtures, but have just one win on the road, at Norwich.
Mourinho added: "At home, five games, five victories and away, five matches, one victory, two defeats.
"We can also say that we played away in very difficult stadiums.
"We played at Manchester United, Tottenham, Newcastle and Everton, we played in probably the most difficult places to go, but I have to analyse it by another perspective which is the perspective of why we don't do better, and why especially on Saturday because that was the worst."
Mourinho has promised changes against Schalke, having made wholesale changes in the matches with Arsenal and Newcastle.
He added: "I will make changes. Not 11 changes because that would be too much, but changes for sure.
"I don't know my best team because we have this kind of performance.
"I know basically which is my best team but it depends on the way we perform because performances and results push you for decisions."