Victory for the Red Devils - coupled with a draw or defeat for Valencia against Juventus - would seal their place in the knockout rounds, although Young Boys are also in need of a win to rescue any lingering hopes of playing European football after the New Year.
It must feel like a sense of one step forward, two steps backwards for Manchester United at the moment.
It was a game very few people expected them to win - and indeed a game they were slightly fortunate to win - but it blew Group H wide open and has left the Red Devils on the brink of qualifying for the last 16 with a game to spare.
However, the highs of that night in Turin already feel like a long time ago; United were quickly brought back down to earth when they were outclassed by Manchester City in the derby four days later, while their return to Premier League action after the international break was hardly inspiring.
Saturday's goalless draw against Crystal Palace - in which the Eagles had plenty of chances - leaves the Red Devils closer to the bottom of the table than the top, with 13 points separating them from 20th-placed Fulham and 14 separating them from leaders City.
Boredom amongst supporters is arguably the most damning indictment of United's current form, which has been the definition of inconsistent with four wins, four draws and four defeats in their last 12 matches across all competitions.
Mourinho's side have even been inconsistent within matches, often relying on comebacks in the second half to get anything out of them - as they did against Juventus on matchday four.
The triumph in Turin leaves United two points behind Juve and two above Valencia, which means that victory coupled with Valencia failing to win away to Juve would see United through to the knockout stages - but anything other than all three points would open the door for Valencia ahead of the two sides meeting at the Mestalla.
It is actually those away European days which have provided United's highlights in this competition so far; the Red Devils' two wins came at Young Boys and Juventus, whereas they are yet to even score a Champions League goal at Old Trafford this season - the first time since 2005 that they have failed to find the back of the net in successive Champions League home games.
United have never gone three European home games in a row without scoring, but they welcome Young Boys in the midst of their longest European home winless run since 2011-12 - three matches including their defeat to Sevilla last season.
Young Boys should be the easiest opponents in this group, though, so anything other than a win on Tuesday would surely heap more pressure on Mourinho's shoulders.
Recent Champions League form: WDLW
Recent form (all competitions): LWWWLD
Young Boys are still very much at the stage of their European development that a trip to Old Trafford is treasured rather than targeted as a realistic source of points, even accounting for United's current struggles.
The Swiss outfit are taking part in the group stages of the Champions League for the first time, having ended a 32-year wait to win their domestic title last season.
United were their first opponents in the Champions League proper too, although even their synthetic pitch could not even out the gap in quality between the two sides as the Premier League outfit ran out comfortable winners.
A defeat to Juventus followed on matchday two before they picked up their first ever point in the competition at home to Valencia, although the reverse 3-1 loss in Spain leaves Gerardo Seoane's side in a precarious position.
Only a win will do at Old Trafford if Young Boys are to maintain any hope of at least finishing third and continuing their European journey in the Europa League next year, and even if they manage that then they would still need to beat Juventus to overtake Valencia.
The tournament debutants are likely to arrive already consigned to their fate, then, but they will be determined to enjoy their evening and should be able to play with the shackles off - something which cannot be said for United recently.
Young Boys are also in a winning habit domestically even if not in Europe, and their form in the Swiss Super League so far this term suggests that this will not be the last we see of them at Europe's top table.
Seoane's side have dropped just five points from the first 45 on offer to open up a mammoth 16-point gap at the top of the table after only 15 games, while also boasting the best defence and attack in the division by some distance.
The Champions League clearly represents a significant step up in class, but a team which has won 17 of 26 matches across all competitions this season can never been completely disregarded.
Sunday's match against second-placed Basel - whom they have already thrashed 7-1 this season - may be of greater importance for Young Boys, though, particularly considering they have won just one of their last 16 European away games and have only won two of their last 14 home or away.
Recent Champions League form: LLDL
Recent form (all competitions): WWWLWW
The Ivorian defender has not featured since being hauled off after just 19 minutes of his side's match against Newcastle United on October 6 and was left out of the squad entirely against the Eagles.
Romelu Lukaku returned to the starting lineup at the weekend but drew another blank to stretch his drought at club level to 11 games, leaving Marcus Rashford and Alexis Sanchez hopeful of a start through the middle.
Man Utd possible starting lineup:
De Gea; Young, Smalling, Bailly, Shaw; Fellaini, Matic, Pogba; Lingard, Rashford, Martial
Young Boys possible starting lineup:
Von Ballmoos; Mbabu, Lauper, Von Bergen, Benito; Fassnacht, Sow, Aebischer, Sulejmani; Nsame, Assale
Head To Head
The reverse fixture in Switzerland was the first ever meeting between these two sides, with United running out 3-0 winners on matchday one.
Young Boys have faced English opposition on seven previous occasions in total, though, winning just one of those matches - against Tottenham Hotspur in 2010-11.
We say: Man Utd 3-0 Young Boys
United are certainly not setting the world alight at the moment, but they are a still a class above Young Boys and should cruise to victory fairly comfortably on Tuesday night. Should results elsewhere also go as expected then that will be enough to see them through to the last 16.