Arthur injured his groin while playing for Brazil and hasn't played since Barcelona's win at Atletico Madrid at the start of December. The club had never revealed the severity of the problem but have now confirmed he will be out for another three weeks.
The midfielder will therefore miss Saturday's game against Espanyol and will also sit out the Spanish Super Cup which is taking place in Saudi Arabia next week.
Ter Stegen, meanwhile, is undergoing treatment on a knee injury. Barcelona have not said how long he will be sidelined for but sources close to the player said he will almost definitely miss the Espanyol game.
A decision on his fitness for the Super Cup -- which features Barcelona, Atletico Madrid, Real Madrid and Valencia -- will be made closer to the time.
The German goalkeeper, who became a father for the first time over the weekend, will be replaced in the short-term by Neto. The Brazilian has made just one official appearance for Barcelona since signing from Valencia for €30 million last summer.
Ousmane Dembele is the only other long-term absentee. The France forward is due back from a hamstring injury at the start of February.
Barcelona return to action in La Liga at Espanyol on Saturday before flying to Saudi Arabia, where they face Atletico Madrid on Jan. 9 in the semifinal of the Super Cup.