Milan had given the Premier League leaders until Friday to accept the club's bid for the want-away striker, announcing the capture of Lopez on loan soon after the deadline expired.
"We were ever so close with City," Milan Managing Director Adriano Galliani told Milan Channel. "If someone had asked me at 17:30 GMT who would be coming, I would have said Tevez. But we've left things well with City and the misunderstandings have been overcome.
"The market opens again in four months, so perhaps it will be just a few months of separation with Carlitos. I did everything I could to bring him in. It was heart in the mouth stuff right up until the deadline."
"I kept my promise to Maxi and when we had not reached a deal with City, we stamped his contract with the league. Maxi will be a useful player for us. He can also play in the Champions League."
Lopez, 27, who played alongside Tevez in Argentina's youth teams, scored 22 goals in 66 games for Catania following stints with Gremio, FC Moscow, Mallorca, Barcelona and River Plate.
