Tuesday's latest reverse meant Celtic missed the chance to usurp Hearts at the top of the table.
Motherwell were worthy winners at Fir Park and could have inflicted a far more embarrassing scoreline on Nancy's men but for wasteful finishing in their first home victory over Celtic since 2013.
Ibrahim Said stooped to head in his first goal for the club before Elliott Watt capitalised on a mistake from Kasper Schmeichel.
Tuesday's defeat was a fresh setback for the Hoops after back-to-back wins but Celtic manager Nancy said: "I'm confident with where we want to go. This was a tough game. But I think that we’re going to get better."
The Frenchman, a former Columbus Crew head coach, added: "In terms of pressure, it doesn’t change the way I think about this because we know that we have to do well. But for the moment, this is the reality and I knew that it could be like that.
"It was a tough, tough night. We faced a good team, collectively really good. I knew that this could be a bit difficult because of the way they play and where we are at this moment."
The loss to Motherwell left Celtic just three points ahead of arch-rivals Rangers heading into Saturday's clash.
Nancy, asked if the Glasgow derby was a defining match for his tenure at Parkhead, replied: "No, not at all. Each game for me is important, and for the team is important.
"I don't think about my future, I think about what I do now and what I need to do to improve the team."