Mourinho was sacked months after winning the Premier League title at Stamford Bridge, and Hazard said he felt partially responsible.
"I don't regret many things in my career, but that I have not been able to work with Mourinho at Chelsea anymore is one of them.
"We had a team that could win more trophies. We just ended up in a negative cycle.
"I sent him a message to say I was sorry he had gone and that I was sorry. We'd enjoyed all that success together, but this time 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 then I performed less."
Hazard racked up 14 goals and 10 assists in the Premier League as Chelsea won the title in 2014-15, but the following season, he produced just four of each, with all of his goals coming in the final five matches of the campaign.
Mourinho was relieved of his duties in December 2015, with the team 16th in the table and having lost nine times in 16 league matches.
Hazard said his drop-off in performance was a result of asking Mourinho for extra holidays, from which he "came back totally out of shape."
Mourinho is now Manchester United boss and is under pressure at Old Trafford after a poor start to the season.
The Special One has also come in for criticism for United's uninspiring style of play, with the team often adopting a conservative approach and failing to convince or entertain in the final third.
Football writer Liam Canning and ESPN's Alex Shaw are among his critics:
Hazard defended his former mentor, however, per HLN's Kristof Terreur:
Kristof Terreur 📰 @HLNinEngeland
EH: “The last season under Mourinho was not enjoyable. We didn’t win, we got into a sort of routine, training without having fun, it was better for all parties that we parted ways. But if I’m asked now about one coach with whom I want to work again, I would say Mourinho."
The forward's candour is hardly surprising. As Bleacher Report's Gianni Verschueren noted, Belgian players are known for being honest when speaking to the media while on international duty.
He and Mourinho aren't likely to link up again anytime soon given a third spell for the Portuguese at Chelsea seems unlikely and with Maurizio Sarri doing a fine job of coaxing the best out of Hazard at Stamford Bridge.
Hazard is only 27, though, so it's possible he may get his wish at some point in his career.