It's both startling and somewhat unfair, the degree to which Barcelona are treading along a knife edge at the Camp Nou. If they eliminate Manchester United from the Champions League, whether by hook, crook, a goalless draw or a champagne fest of scintillating, inspired football, they will be about three good performances away from their third trophy Treble.

That their opponents Tuesday happen to be Manchester United -- led now by Ole Gunnar Solskjaer, their mythical added-time goal scorer from 1999 -- on a date that's extremely close to the 20th anniversary of United becoming the only English team to join that small, fabled list of Treble winners is just one of football's quixotic little twists of humour.

The fact that Tuesday begins with the scoreline 1-0 to Barcelona, with Solskjaer on the Camp Nou bench, as time ticks away, as it was when 90 minutes were up against Bayern in 1999, is one more delicious twist. But United aren't Barcelona's only rivals: History and destiny are, too. Those three hypothetical "good performances" that, in my opinion, could win them the Treble would need to be a big, firm win in the semifinal first leg, a winning performance in the showpiece UEFA final on June 1 at the Wanda Metropolitano in Madrid and, in between, a display with enough muscle and flair to defeat Valencia in the Copa del Rey Final next month.

They're close to their first trophy already. Barcelona can count victory in La Liga as being right there, the finish line a mere formality. Routine, attentive work will get them over that line, and they'll win their 26th Spanish championship.

Valencia, in my opinion, have a starting XI, a squad and a coach capable of ruining this Blaugrana dream in the Copa del Rey. Because while the Catalans, current Copa holders and winners of four in a row, will enter the Benito Villamarin final on May 25 as favourites, Marcelino's side is full of talent, pace and the odd warrior. Potentially, they are a gremlin in Barcelona's machine.

It all means that while there are oodles of hurdles left, starting with Anthony Martial, Marcus Rashford, Paul Pogba & Co. on Tuesday, the Spanish champions are in a tantalisingly beautiful position.

The Treble must consist of a club winning its premier domestic league, the principal domestic cup competition and the Champions League. No exceptions, no asterisks: This is the pinnacle of continental football.

Only seven clubs have managed it. In chronological order: Celtic, Ajax, PSV, Manchester United, Barcelona, Inter Milan and Bayern Munich. Only Barca have done it twice, meaning that three men -- Sergio Busquets, Gerard Pique and Lionel Messi -- stand to become the only footballers in history to complete a hat trick of this mythical and viciously difficult threesome.

Barcelona are so close, yet still so far, from a historic third Treble. Yet the mood around the club is that anything less than the league, Copa del Rey and Champions League would be disappointing. Simon Stacpoole/Offside/Getty Images

What I mean when I say it's all on a knife edge is this: It wouldn't take much to ruin this mission. Should Solskjaer produce another against-the-odds victory to eliminate his Camp Nou opponent despite trailing by a goal, then notwithstanding the fact that three Barca players already have more Trebles than anyone other than a handful of their own former teammates, something of a cold glare will be turned on the Barca era since 2015.

It's unfair, but it's true. The fact that Barcelona have performed poorly at this stage of the Champions League in each of the past three seasons, losing the past three quarterfinals by an aggregate margin of 10-6 and failing to score in four of the six games, has left a stain on their recent Spanish dominance. It's an invisible stain if you're the guy who has had to coach them to repeat Liga and Copa victories. It's invisible to guys such as Messi, Ivan Rakitic, Pique, Busquets and Jordi Alba as well, players who know precisely how hard they've had to work to rack up eight trophy wins since they sealed their last Treble four years ago in Berlin.

How many clubs would sell their soul for eight trophies in four years? Your club? Maybe even Man United right now?

Turn that stat on its head, and most players would swap a full head of hair and all their image rights if only they could win four trophies in eight years. But for those critics who are now of the mindset that greatness lies only in winning the Champions League or retaining the "Cup With the Big Ears" (as the European Cup is often nicknamed), Barcelona's stain is more indelible than invisible. The power of conquering the continent has grown so alluring to sponsors, advertisers and club marketers -- not to mention so lucrative when you factor in the Champions League revenue, the prize money and your share of the television pot -- that those who say "winning the domestic league is more important" have dwindled down to hardcore supporters and players alike: the true traditionalists.

Clearly, the majority of Barcelona's squad still think that way. They believe that the 24-karat test of grit, resourcefulness, professionalism, resilience and, in their eyes, "greatness" lies in repeatedly dominating your own nation. Hence the choke grip in which they've held La Liga for most of the past 14 years.

But if United pop up with a surprise this week and eliminate Ernesto Valverde's guys, or if whoever lies in wait in the semifinals prevents Barcelona from reaching the Wanda Metropolitano on June 1, then I'm guessing a huge section of the Camp Nou fan base and the media who hawkishly cover every waking second of every Barca day would judge the four years since the defeat of Juve in Berlin to have been "a disappointment."

Whether fair or unfair, correct or incorrect, credible or incredible, it wouldn't matter to some. Not only is the Champions League regarded by many modern fans as the be-all-and-end-all, but also the concept of a third Treble, which is genuinely within reach, is so intoxicating that ending the season with anything less would be enormously deflating.

I don't see it quite that way.

What Barcelona have managed since the Treble win in 2015 -- given the trauma of Xavi, Andres Iniesta and Neymar all departing, not to mention the stress of how to properly replace such legends, and the fact that their truly great players stubbornly keep on aging -- has been quite remarkable. It's history book stuff. But the fact remains that from the point when Messi promised that he'd do his absolute utmost to return that "beautiful, historic trophy" to the Camp Nou, when he assumed the captaincy last summer until now, Barca have put themselves within three or four really good performances of not only making history but also becoming a statistic as demolishing to all competition as Real Madrid's European Cup-winning record is in its own way.

This tie, for as much as United have players who can threaten and for as much as Barcelona aren't on their most fluent, most dangerous form right now, isn't the moment to look history and destiny in the eye and flinch. This is the occasion to show that, with domestic superiority once more attained, the lessons of the past three seasons truly have been learned.