The Swedish striker played for Milan between 2010 and 2012, scoring 56 goals in 85 games in his first season. Juventus star Ronaldo then played for Real Madrid.
The two had previously been in Europe – when Ronaldo was in Madrid and Ibrahimovic in Paris Saint-Germain – and also on the international stage at the 2014 World Cup. They also share a former club in Manchester United.
"You have to suffer"
Ibrahimovic helped Milan win the 18th Serie A title in 2011 – the club's last major title – but while Juventus and Ronaldo are trying to defend their championship, the 38-year-old Swedes job is to save a season this time The time has come to be marred by the mediocrity for the Rossoneri.
Before the game against Sampdora on Monday, Milan were 11th in the Serie A table, eight points from a place in the Europa League and 14 points from a place in the Champions League.
"You have to suffer: If you don't know how you suffer, you can't be the best," added Ibrahimovic. "I'm getting meaner. I also expect a lot from my teammates.
"I only saw the results of Milan from a distance, but I don't know what hasn't worked so far."
Ibrahimovic said since the forward's contract with LA Galaxy expired in December, he had "received 38 more offers than 20".
"I was looking for a final push of adrenaline to give it all," he added.
"We are delighted to welcome a unique player like Ibra back," said Zvonimar Boban, Milan soccer director, who also played for Rossoneri.
& # 39; We are very confident of the impact it will have on the team and the atmosphere, and it is clear that results are needed.
"I don't want to forget the game in Bergamo, which was an unacceptable loss. We shouldn't hide behind Zlatan's broad shoulders. We have to change direction this season and hope that his presence will help us."
"I feel ready for AC Milan"
Since the separation from Massimiliano Allegri in 2014, the club has had nine different managers, including stations of the former players Clarence Seedorf, Filippo Inzaghi and Gennaro Gattuso.
Recent history has been quite a crash for a club that won five championship titles in the 1990s and two Champions League titles in the 2000s.
"You can't change history – AC Milan will always be AC Milan," said Ibrahimovic.
"There was a chance to come back here … but I still didn't feel ready after my injury and I wanted to continue playing in Los Angeles because I was fine.
"I went to America to feel alive. After two seasons in the MLS, I feel more than alive. I feel ready for AC Milan."