Corinthians are reported to have made a 15m bid for Pato, which could even involve a swap with midfielder Paulinho.
Galliani said: "An offer has arrived for Pato. Basically, in defence and midfield we will not be signing anyone. If Pato and Robinho remain, or if only one leaves, then we won't bring in any replacements.
"We believe the right number of strikers to have is five, as if two new ones arrive, then there won't be space for M'Baye Niang."