Literally it's not that we completely depend on the past to make out predictions, basically it's supposed to be that the past was to give us a insight of a possible outcome when certain teams meet and not an absolute to an expectation from both teams, but some persons makes it the absolute and. That could be reason for some of their losses this season. Chelsea has got a very fierce record with Manchester city of which entails mostly an epic game between the two sides , so for fixtures like this expectations usually are kept at its minimal.
I agree with @Frankolala, the past result do give an outcome of the next game and it works but not 100% perfect, since this game between Man City vs Chelsea is one of the top game this weekend in the EPL the past result can work on this but this time around things have changed and we know they played 0-2 in favor of Manchester City, in their first leg, that doesn't mean City will win again. Let's face the reality, Man City are really in one of their worse form since Pep took over and the Chelsea side are better this season and is like the both teams had a switch of form. As of last season Chelsea was suffering like City now and City was on fire, if you check what the bookies gave both teams in this encounter you'd see that City are the favorite even with their poor performance but I believe if anything concerning a win would take place Chelsea are going to be the winner, I'll be going for a 1-3 for the Blues.
I do look at past games, but not h2h, because head to head doesn't give good outcome if you ask me, I look at recent games, like how the teams played in the last 5-10 games, if you look at that then you are going to see how the team is doing and I am pretty sure that we are going to end up with a good result without a doubt. I am not entirely sure how this could end up being different all that much, but we need to be considering how things could change without a doubt.
If people could put their analysis on just how the teams are doing right now, that would be better, instead of focusing on head to head. Think about it, these teams meet maybe twice or three times a year, so if you look at last five games, you are looking into not just last season, but also season before that, makes no sense.