The football season is long, and many themes are revisited over the course of it. Many lessons are learned—some unlearned—and while a team will evolve over the eight or nine months it will only become as strong as its weakest characteristics allow. It may even overcome them for a while, but not forever. This is a fact of life, nevermind football.

Themes revisited. From January 25 to February 11 Barcelona won only two of six matches. They fell 10 points back of Real Madrid in La Liga and were largely written out of the title race with three months to go. At the time, many questioned the depth of squad available to Pep Guardiola. The manager had an all-world starting XI, the thinking went, but beyond that there were some shortcomings—shortcomings that were costing the club points.

That theme—Barcelona’s lack of depth—was all but forgotten over the next eight weeks as the Catalan giants rattled off 11 league wins in a row and progressed to the Copa del Rey final and Champions League semifinals. But it has come to roost once more, and when all is said and done it may be the defining theme of their season.

Barcelona never really had an answer when striker David Villa was lost for the rest of the campaign to a broken tibia in late 2011, and while Javier Mascherano has deputised effectively in central defense he was never going to be the ideal replacement for Gerard Pique, who because of attitude problems, form problems and undisclosed problems has been in and out of the lineup much of the season.

Inadequate squad depth is a dangerous thing for a side with ambitions in several competitions, even one as talented and front-loaded as Barcelona. Its initial symptom is team fatigue, and once that sets in a general lack of quality in the players called up to give the tired ones a rest can significantly depress a record.

Never was this more apparent than in Saturday’s Clasico, won 2-1 at Camp Nou by Real Madrid. The seven occasions on which Barcelona had previously dropped points away from home should have been sign enough that the old standard was out of reach this time around, but in losing at home to their existential rivals the Catalans hit a low they hadn’t experienced since Guardiola arrived in 2008.

With Alexis Sanchez unable to play the full 90 minutes Guardiola had to pick between Pedro and Christian Tello to replace him. Pedro, himself, has been limited by injury this season and, when healthy, has only been able to contribute three goals in La Liga, so the manager selected Tello. The drop-off was obvious.

Guardiola also opted to start Cesc Fabregas from the bench. The former Arsenal man hasn’t played more than 30 games in a season in four years and has clearly petered out after a strong start to his first Barcelona campaign. But because he sat Andres Iniesta had to be deployed further upfield, which necessitated a start for Thiago Alcantara. Thiago is a fine player and should certainly not be blamed for anything that went wrong on Saturday, but he’s hardly Iniesta, and for Barcelona to maintain their standard they need Iniesta, Messi and Xavi playing at or near 100 per cent and in their best positions.

Xavi, against Madrid, was withdrawn after 70 minutes. This would have been unthinkable in November or December, but with 47 matches under his belt this season the 32-year-old is obviously finding each new step a little heavier than the last.

And Messi, football’s Energizer Bunny, was as ineffective against Madrid as he was midweek at Chelsea. He’s played 54 matches already this season—most of them for the full 90 minutes. That’s four successive seasons of at least 50 matches and five of at least 40. Add in international appearances and it’s simply an unsustainable amount of football.

That Barcelona have been able to maintain such a high standard for so long speaks to both the all-world talent of their top players and their individual commitments to fitness. And while three years of utter domination constitutes a rare dynasty in the realm of football there’s no reason to believe it couldn’t go on even longer with the acquisition of adequate depth.

It’s a theme that has been attached to the Catalan giants all season but became all the more pronounced in the past week. Without adequate depth Barcelona risk falling off the pace even sooner next campaign than they did this. They risk burning out their best players, especially Messi, well in advance of their best-before dates. This is a theme they can’t afford to revisit.

Follow Jerrad Peters on Twitter @peterssoccer