Monreal left La Real for the Gunners in August, but only after convincing Emery to allow him to depart - this despite the Premier League season having already kicked off.
The veteran defender admits he had a good relationship with the former Gunners manager and was sorry to hear of his demise. Emery was sacked by Arsenal on November 29 - less than 18 months into the job.
Along with Emery's axing, Monreal, who spent over six years with Arsenal, also concedes he's disappointed to see his old club struggling this season. With the Gunners, the 33 year-old won three FA Cups under former manager Arsene Wenger.
He told Marca: "Things are not going as they thought. I want to be optimistic, because I know what potential there is. I am sad that they did not finish taking that step forward that they were supposed to take this season.
"And also because I had a very close relationship with Unai and, when you have that personal relationship, you want to see him succeed and achieve the goals, because he and his coaching staff are also great professionals."
- updated December 9