If the Champions League group stages were run like most other "normal" leagues, we would all be very familiar with the tiebreakers. Also, Chelsea would be sitting very pretty, with by far the best goal difference in the group (+8) thanks to our matching 4-0 wins over minnows Maccabi Tel-Aviv. Both Porto and Dynamo Kiev are only on +3.
But the Champions League doesn't play by the usual rules, and that does make sense given that the group stages consist of just six matches rather than a full league season. So instead of goal difference and goals scored, the tiebreakers rely on head-to-head results and goals.
We could've avoided these complications for the final matchday of Group G had we not lost in Porto or drawn in Kiev, or if Porto didn't lose to Dynamo on Tuesday. We can still avoid complications simply by winning our final match, at home to Porto. First place, no tiebreakers, no worries. Anything other than a win opens the door for craziness.
Let's break it down.
All of this is predicated on the assumption that Dynamo Kiev will take care of business against Maccabi in their final game. The Ukrainian side are playing at home and should have no problem doing so. Anything other than a win for them ensures that both Chelsea and Porto make the knockout rounds, while Dynamo drop into the Europa League. First place in the group would then be determined by whoever wins the match at Stamford Bridge, with a draw giving Porto the head-to-head edge thanks to our loss in Portugal.
If Dynamo do win as predicted, the loser of the match at Stamford Bridge drops into the Europa League, while the winner goes on to win the group.
Lastly, if Dynamo do win and the match at Stamford Bridge ends in a draw, we reach a dreaded three-way tie on eleven points. This is a fairly unusual scenario, so we have to crack open the rulebook to see what tiebreaker madness UEFA would have in store for us. Here's Article 17.01 with the details.
The first few criteria concern themselves with "matches played among the teams in question", which removes our good results agains Maccabi. But it removes two wins from everyone else, too, leaving all teams tied on 5 points (1 win, 2 draws, 1 loss) once again. Criterion a) is out.
Criterion b) would then decide the group. Without the Maccabi wins, our goal difference is ±0, while Porto's goal difference is a (-1). A draw would leave those goal difference numbers unchanged. Dynamo, having already played all four matches in the Maccabi-less mini-league, already have their goal difference set at +1. They would thus win the group (+1), Chelsea would finish second (±0), and Porto would end up in third place (-1) and in the Europa League.
So, Chelsea need a win or at least a draw to qualify. Or we need a very unlikely Dynamo Kiev loss or a draw. Plenty to worry about for the final match-day then, coming up in a fortnight, after a trip to White Hart Lane and a home match against newly promoted and struggling AFC Bournemouth.