Ibra was in the stands as Pato helped Milan to a 4-0 victory over Lecce.
The Brazil striker said: "I am happy about his arrival.
"Every one of us waited for it. We can win everything with him. A great champion has arrived and now the Coach will decide how to make us play.
"It was wonderful to get off the mark so quickly, and also Milan played well. Those who came on also managed to do well.
"We must still improve, but I am sure that we'll have a great season."