The Serie A side face Ajax next in a showdown for second place in Group H, and the Colombian defender is already anticipating the clash.
"Now we face Ajax at home and will do anything to win," Zapata told Sky Sport Italia.
"During these last few days has been difficult for us. We have now given an answer in Europe and we hope to do the same in the league.
"I'm happy to have scored today [against Celtic] it was a great win and we deserved it."
A point for Milan against the Dutch side will take them through to the knockout stages.