Madrid - Philippe Coutinho scored a crucial away goal but Barcelona have work to do to avoid a surprise exit from the Copa del Rey after losing 2-1 to Levante on Thursday in the first leg of the last 16.
Lionel Messi, Luis Suarez and Marc Andre ter Stegen were among several key players rested at the City of Valencia Stadium, where Levante took advantage as Erick Cabaco and Borja Mayoral gave them a two-goal cushion.
Coutinho, however, making his first start in almost a month, converted an 85th-minute penalty to leave Barca in touch heading into next week's second leg at the Camp Nou.
After missing much of November through injury, Coutinho has struggled to regain his place in Ernesto Valverde's preferred starting line-up but 90 minutes, and a goal, will have done his confidence no harm.
Valverde was blamed last season for Barca's shock Champions Leaue exit, the theory being that he failed to rest key players in the Copa del Rey and La Liga, both of which they won.
He seems keener to adjust this term as a much-changed line-up included a debut for central defender Jeison Murillo, a January loan signing from Valencia, as well as youngsters like Chumi Brandariz, Juan Miranda and Carles Alena.
This was a difficult introduction, however, with Murillo beaten for Levante's second goal while Miranda was replaced at half-time and Chumi followed 13 minutes later.
The front three were more established, as Ousmane Dembele, Malcom and Coutinho aimed to make up for the absence of a recognised striker.
Instead, Levante took the lead after just 185 seconds as Ruben Rochina's in-swinging cross from the left met the head of Cabaco, who climbed above Sergio Busquets and headed in.
Barca were struggling to cope with the aggression of their opponents and in the 18th minute they were two down. Emmanuel Boateng's reverse pass set Borja free, who nipped between Murillo and Chumi.
Murillo tried to make the block but Borja's shot beat both him and Jasper Cillessen, before nestling in the far corner.
Levante could have made it three but Borja fired wide and Jose Luis Morales, who only had Cillessen to beat, scooped his finish over the bar.
Valverde had turned to his bench as Sergi Roberto and Clement Lenglet came on for Miranda and Chumi while Denis Suarez, linked with a move away from the club this month, was given 23 minutes to instigate a comeback.
He did the job, a weaving run inside from the right drawing a clumsy challenge from Coke. Coutinho made no mistake.
Earlier, Sevilla put one foot in the quarter-finals after winning 3-1 away to Athletic Bilbao.
Mikel San Jose pulled Bilbao level at San Mames after Nolito's opener but second-half goals from Andre Silva and Wissam Ben Yedder mean Sevilla will be heavy favourites next week.