Assuming that Liverpool beats Napoli, then the tiebreak rules are:
1. Number of points obtained in the group match among the teams in question --> Napoli 3, Liverpool 3
2. Ggoal difference from the group matches played among the teams in question --> Napoli is at +1 and Liverpool at -1. So if Liverpool wins by 2+ goals, they win. If they win by one goal, we go to the next tiebreaker.
3. Number of goals scored in the group matches played among the teams in question --> To reach this tiebreaker, Liverpool has to win by 1 goal. So both teams are equal here.
4. Number of goals scored away from home in the groupe matches played among the team in question --> If Napoli scores a goal at Anfield, then they have an away goal and Liverpool has zero. So Napoli goes through. But if Liverpool wins 1-0, both are equal here.
5. Goal difference from all groups matches played --> Napoli is currently at +3 and Liverpool at +1. If Liverpool wins 1-0, both are equal at +2.
6. Goal scored from all groups matches played --> Napoli is at 7 goals scored and Liverpool at 8. So with a 1-0 win, Liverpool goes through.