For all the promise they showed in the second half, the damage was done in a lackluster first half when they allowed Tottenham to race into a 2-0 lead.
Man U Manager Sir Alex Ferguson had made it clear that he will not tolerate slip-up's in Europe this season, so this is a fixture United will have targeted as three points.
For Cluj, this is their Champions League final. The day they welcome a European giant to their stadium, is one in which they will not just roll over and accept defeat.
Here are six predictions of how the action will unfold in Dr. Constantin Radulescu Stadium.
Just when United's injury crisis in defence appeared to have abated, Nemanja Vidic was forced to undergo surgery on his knee, leaving the Red Devils short of cover at the back.
Rio Ferdinand is coming off his own injury and looked tired on Saturday, while Ferguson has already stated his belief that over-working Patrice Evra has contributed to his poor form.
In midfield, Paul Scholes and Michael Carrick have played much more than Ferguson would've liked, so Fergie may see this as a chance to rest a few of his veterans.
I expect Alexander Buttner and Scott Wootton to come into the defence, with Wootton having shone last week in the Carling Cup.
Tom Cleverly and Anderson will get another chance to impress, while Shinji Kagawa should provide the creative spark.
Darren Fletcher will continue his journey back to fitness by anchoring the midfield and protecting the inexperienced back four.
Wayne Rooney will come into the team, at the expense of Robin van Persie, and get a chance to resume last year's fruitful partnership with Danny Welbeck.
When it comes to who will play in goal, your guess is as good as mine, as United has changed keeper before each of the last five games.
Romanian fans are known for being passionate, so the Cluj fans will relish the visit of a European giant. This will be their biggest game of the season, so fan support will spur Cluj to a fast start.
This will be helped by the fact that United have been very slow out of the gate this season. In eight games so far, they have fallen behind in five of them.
Some of the young players will never have experienced an atmosphere like this before and could be overawed by the occasion.
Modou Sougou and Rafael Bastos are two players who could trouble United, having developed a good understanding in their short time together.
Both players have the pace and power to trouble the defence, so United will have to be careful to not leave them exposed to fast counters.
While Cluj are a threat going forward, they can be exposed defensively, as highlighted by the fact they've struggled to keep clean-sheets this season.
Their attackers are talented but not renowned for their hard work, often failing to track back and help out the full-backs.
If, as expected, United line up in a narrow diamond, like against Newcastle, Buttner and Rafael will be tasked with providing the width.
Both players can score and provide assists, and if United are to win, these two will need to exploit the holes at that back and make the most of their opportunities.
Kagawa has the nous to put them in space, while Anderson and Cleverly's forward runs will occupy the midfielders.
Expect both players to get on the stat sheet, either with goals or assists.
United are not known as a dirty team, but they were the second-most penalised team in Week 1 with 20 fouls.
The only team that committed more fouls were Cluj with 22.
If, as expected, United build up a lead, the Cluj players could lose their cool out of sheer frustration.
On the flip-side, players like Buttner, Rafael and Welbeck are prone to rash challenges, which could see them fall foul of Cluj's pacy attackers.
A large number of Cluj's players have played in the Portuguese League, where diving is an art, as highlighted by both Nani and Ronaldo. You add that to the passionate home crowd, and it could spell trouble.
United's players will need to stay on their feet and not give the referee any chance to be swayed by the crowd.
What a difference a year makes.
This time last season, Hernandez was second only to Wayne Rooney in the pecking order at Old Trafford.
Now he finds himself fourth.
The signing of RVP and the rise of Welbeck has seen Chicharito slip down the order, limiting his chances to fight his way back.
He lacks the all-around game of the other United strikers, but his nose for goal is surpassed only by RVP.
He won't start, but I predict he'll come on with 25-35 minutes to go to relieve Rooney, and once unleashed, he will do serious damage.
Cluj's back four can be exploited by pace, especially when they are forced to chase a game, and Hernandez is the perfect player to take advantage of this.
As Cluj press forward, expect him to sit on the shoulder of the last defender, waiting for a chance to pounce.
He may get caught offside more often than not, but he will exploit these gaps to the tune of two goals to kill the tie.
Watching United this season has been like watching a pick-up basketball game, as the game and momentum flies back and forth endlessly.
Against Spurs, Nani got United back into the game only for Tottenham to race away and restore the two-goal lead. Almost immediately, United struck again, as Kagawa brought the score back to 3-2.
In three minutes, the exact time it took for me to use the facilities and miss the action, the game had swung back-and-forth, going from 0-2 to 2-3.
On Tuesday, I predict United will fall behind, take control and open a 4-1 lead before taking their foot off the pedal and allowing Cluj a second.
This second will ensure a nervy end to the game, but United will walk away with the three points.