Barcelona could only draw 1-1 with Celta Vigo in the first leg of their Copa del Rey round of 16 fixture on Thursday night.
Without Lionel Messi and Luis Suarez in the team, Barca found it hard to finish off the tie.
Barca B striker Arnaiz opened scoring for the Catalans, scoring in his third successive cup appearance, when he finished off a low cross from Andre Gomes.
Their lead lasted only 16 minutes, as Pione Sisto levelled with a fierce finish.
Ousmane Dembele, who came on for the final 20 minutes of the clash, was unable to score the winning goal for Barcelona.
The club's record signing made his long-awaited return from injury, after being sidelined with injury since September.