it is impossible to say for the following reasons:
the two teams cannot meet in the group stage which means that there will be 16 teams left in the competition by the time they can meet in the first instance. As the odds of either side progressing to the last 16 can only be quantifiable on an empirical basis then the probability of them meeting will still be based on an empirical footing.
If it were equally likely to progress through each round they then there is around a 71% chance that they would meet. This is because as they progress through the stages there is an increase chance of meeting. Over five years it comes out at around 18% but bare in mind that this does not include the empirical odds that are attributed to each match. The likelihood of Liverpool progressing out of the group stage is greater than say Olympiakos but less than Barcelona so the above figure is flawed.