Milan recovered from a league defeat to Verona at the weekend to beat PSV 4-1 on aggregate, and Balotelli has warned that the players must now improve in Serie A.
"We're delighted to have played a great game," Balotelli told Sky Sport Italia. "We did so in the first leg too and we were really keen on this because the Champions League is important. If you can't cope with pressure, then you'd be better off not playing football.
"We've passed the test, but then if on Sunday we go and play badly, you'll say we're immature. I think we are mature, even before the Verona game. Since last year, we're well on our way to becoming the true Milan.
"We need to grow, like everybody, but slowly but surely we will reach perfection."
Milan will find out their Champions League group stage opponents on Thursday evening.