Defeat at Porto leaves Milan with no points from their opening three games.
Tonali said: "Every match is different, every team you meet is different and must be faced in different ways.
"Today we lacked the spirit of Liverpool, it was not our best match in the Champions League.
"We must continue on the path we took before this match. It did not go as we wanted, we started with difficulty but in the first half we proved to be a team.
"In the second half we suffered together until the episode of the goal, from there it was another game because that thing stays in your head."
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