The Sky Blues are trying for a third consecutive Premier League title, but after 17 games they are 14 points behind leaders Liverpool.
And De Bruyne says Guardiola has been affected by City's fluctuating form.
"Sometimes he's moody, sometimes he's happy," De Bruyne said after City's latest victory.
"But obviously he's going to be a little bit more moody because we are not doing as well as we did the year before. But I think everyone in a job would be the same.
"For the rest, it's the same. When it's not going well, there are going to be more rumours and people are going to talk more about it. It's all fine, everything is normal."