The Belgium international went back to his old club on Sunday for the first time since leaving for the capital in January.
"I left a piece of my heart here and will always be a Cagliari fan," he told Nuovo Sardegna.
"I'm glad that the fans greeted me with affection. I also heard some boos, but I expected some of that.
"At first I was very excited to come back, and it went well for me personally.
"I'm just sorry that my former teammates are going through such a difficult time at the moment.
"I'm sure they will come out of it stronger though."