Speaking to Viaplay ahead of the game, Ferguson said he contributed as he didn't want Ronaldo playing for City.
"I'm excited like everyone else, there was anticipation all day yesterday and waking up in the morning. I speak for all supporters -- it's a great day for Manchester United," Ferguson said.
"A lot of people played their part [in bringing Ronaldo back] and I contributed knowing that really Cristiano wanted to come here and that was important. It worked very well.
"I wouldn't say it's emotional but it's exciting for me and a relief because I couldn't imagine him playing for Man City, I don't think anyone could.
"That's why we took steps to make sure he came here and the club then followed in very well and I spoke to the Glazers and it was done. I think he will make an impact on the young players in particular, he's got the experience to handle the occasion."
Ronaldo spent six seasons at Old Trafford between 2003 and 2009. While with the club he won eight major trophies and scored 118 goals.