Manchester United suffered a major blow to their Champions League hopes on Wednesday.

The Red Devils were always going to have their backs against the wall after being drawn in a group with Paris Saint-Germain and RB Leipzig, but qualification was within their grasp this week.

United had flown out of the blocks in Europe this season with a 2-1 win at the Parc des Princes, before romping last year's semi-finalists RB Leipzig 5-0 at Old Trafford.

Man Utd 1-3 PSG

And despite having lost in embarrassing fashion at İstanbul Başakşehir, United could have wrapped up qualification with a win over PSG after exacting revenge on their Turkish conquerors the week before.

But it simply wasn't to be. PSG ran away 3-1 victors at the 'Theatre of Dreams' with Neymar bagging a brace and Marquinhos finding the net to cancel Marcus Rashford's first-half equaliser.

However, the turning points of the game actually transpired when Anthony Martial missed a glorious opportunity at 1-1 and Fred was given his marching orders for two avoidable yellow cards.

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Group H gridlock

As a result, Champions League Group H couldn't be more finely poised with only İstanbul, who suffered a dramatic 4-3 defeat to Leipzig, having no hope of reaching the second round.

In fact, PSG, Leipzig and United are all level-pegging on nine points with the final round of fixtures at the Parc des Princes and Red Bull Arena poised to whittle down three clubs into two.

It won't come as a surprise that United winning, for example, would secure their place in the second round, but football is never simple and things could quickly turn into a mathematical nightmare.

It's something that United fans will be all-too familiar with as they entered their game with Leicester City on the final day of the 2019/20 Premier League season amdist a complicated top-four race.

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Champions League rules

But there's nothing that can't be simplified with a diagram and that's exactly what's happened on United's Reddit overnight with one fan clearly demonstrating how the order of Group H could shift.

Now, before we introduce you to this cheat sheet on how United could fare, it makes sense that we talk you through the rules on how Champions League groups are decided if two clubs are tied.

1. Points in head-to-head matches among tied teams

2. Goal difference in head-to-head matches among tied teams

3. Goals scored in head-to-head matches among tied teams

4. Away goals scored in head-to-head matches among tied teams

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Diagram of United's situation

Technically this hierarchy of tiebreakers goes all the way down to a twelfth rung of UEFA coefficient, but we won't be needing that here, so check out the Group H graphic ahead of the deciding games:

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So, if United win or draw at Leipzig, they will qualify for the round of 16, but defeat will relegate them to the Europa League even if İstanbul Başakşehir pull off a historic result in the French capital.

And that's the major takeaway here because it means Neymar's second goal at Old Trafford in stoppage time could have massive ramifications, particularly in terms of PSG progressing.

Hanging in the balance

It meant that PSG's win over United was more convincing than the defeat they suffered in the opening week, giving them a superior goal difference for their head-to-head matches.

While an Istanbul win seems unlikely, it means that Thomas Tuchel's men won't have to worry about their own situation if Leipzig start pulling away into a comfortable lead against the Red Devils.

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Besides, making the trip to Germany won't be an easy one for United and Ole Gunnar Solskjaer's future could very well pend on whether they avoid defeat and the Europa League in turn.