A 1-1 draw at Real Sociedad was described as a “miracle” by Barça boss Luis Enrique in what he singled out as the worst performance in his three seasons in charge.
Barça are already six points adrift of Real Madrid ahead of their meeting in El Clasico on Saturday.
Here, AFP Sports looks at what has gone wrong so far this season for the Spanish champions.
Key injuries hurt
Messi rescued Barça from a fifth consecutive defeat on their travels to San Sebastian, but Sunday's draw was the first time Barça have dropped points in the league this season when the Argentine has completed 90 minutes.
Messi's presence has been sorely missed in defeats to Celta Vigo and Alaves and draws with Malaga and Atletico Madrid.
However, he is far from the only key man to have sat out some part of the season so far.
Andres Iniesta has been out for over a month, whilst his Spanish international teammates Gerard Pique and Jordi Alba were also sidelined for a number of weeks.
An often underrated pillar of Barça's success in the past decade, Sergio Busquets is in the midst of his most protracted dip in form since breaking into the first team under Pep Guardiola in 2008.
It is no coincidence that Busquets's struggles have come for the first prolonged period in his career without retired legend Xavi or Iniesta by his side.
His influence in terms of touches and passes completed has wained since Enrique took charge and favoured a more direct style to feed a superstar front three of Messi, Luis Suarez and Neymar more quickly.
However, without the ball Busquets is now being exposed by the lack of support around him as Ivan Rakitic has been forced wider on the right to protect the inexperienced Sergi Roberto, who the departed Dani Alves at right-back.
Whilst Barça have failed to hit the heights of old, the gap between them and Madrid can also be explained by the differing challenges they have faced to date.
The Catalans have already made tricky trips to Sevilla, Sociedad, Athletic Bilbao, Celta Vigo and, despite their current struggles, Valencia, as well as Atletico Madrid at home.
Real's 31-game unbeaten run will be put firmly to the test at the Camp Nou on Saturday, but other challenges await for the European champions with four away trips in five games just as the Champions League returns in February.
Summer signing busts
Barça splurged over €120 million ($127 million) in the summer transfer window in a bid to bolster their squad.
So far to no avail as none of their six recruits have established themselves in Enrique's preferred starting XI.
Despite beating off Madrid to the signature of Andre Gomes, the Portuguese Euro 2016 winner has failed to make an impression in Iniesta's absence, whilst €30 million striker Paco Alcacer has yet to score a goal.
A year ago whilst Messi was sidelined for two months by knee ligament damage, Neymar and Suarez not only ensured Barça didn't lose ground in the title race, but accelerated past Madrid at the top of the table.
Twelve months on and Barça's Messi-dependence has reemerged with the Argentine scoring 10 of Barça's last 14 goals.
Suarez has scored just once in his last five outings, whilst Neymar hasn't scored from open play in La Liga since September.
By AFP's Kieran Canning