The Brazilian returned to the San Siro in the summer after a four-year spell at Real Madrid and came on as a substitute in yesterday's win over Udinese in Serie A.
"It was an emotional experience playing again at the San Siro," Kaka told Milan Channel. "I've been thinking about all day when I was going to come onto the pitch.
"The fans, the welcome, it's something that I'll always keep with me. It's great to know that the fans expect a lot of you.
"I'm missing match fitness and I don't know when I'll be back on form but I'm hoping to make it as soon as possible."
Kaka made over 200 appearances during his first spell at Milan.