Hazard was named the PFA Player of the Year as Chelsea stormed to the Premier League title last season but his form in 2015-16 has fallen well short of those standards.
The Belgian did not score his first goal of the campaign - a penalty against Championship side MK Dons - until the end of January and says he felt guilty about Mourinho's dismissal.
"I sent him the message to say I was sorry he had gone and, well, just that I was sorry," Hazard told the Guardian . "We'd enjoyed all that success together last season, but this time round we hadn't.
"I felt a little bit guilty because I'd been player of the year. I'd been one of the most decisive players, and this year I'd performed less well.
"I hadn't been at the same level. So I sent that text to Jose and he came back to me, wishing me all the best for the future. For a team of champions to go through what we have this year even I can't explain.
"Things have been better recently, but we're still not winning games quite as we used to. No one can put his finger on what's happened at Chelsea."
Hazard ended a wait stretching back to May when he hit the target against MK Dons on Sunday but insists he never allowed his morale to drop during his barren spell.
"It never became an obsession for me to score at all costs," he said. "I've always said that I'm not a big scorer, I'm a worker. But you can't escape you've not scored and it's already January, so last Sunday was a bit of a relief.
"I don't think my morale was down, but it plays on your mind a bit. Everything went so well in the past. This year, not so much. But you have to tell yourself that football is like that.
"You still have to take enjoyment out of what you're doing and things will turn, and my smile has always been there. In good moments and bad. I'm somebody who can laugh even at myself."
Hazard revealed the importance of the support of his former team-mate Didier Drogba as Chelsea he struggled to turn his form around and believes he can become a leadership figure at Stamford Bridge by doing his talking on the pitch.
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"He's always backed me," Hazard said of Montreal Impact striker. "I'm not sure whether he'd ever experienced periods like that in his career, but I've always considered him a bit like a big brother.
"He's there for me, whether it's a text message or a phone call, and he's helped raise my spirits. But I've never doubted myself.
"I've never been one to deliver speeches in the dressing room, like a John Terry, Frank Lampard or Drogba, but I've always tried to lead in my own way on the field: demanding the ball, trying to make a difference.
"The day I'm 100 per cent again, I'm convinced Chelsea will perform better too. It's up to me to raise my level again."