Brazilian star Neymar will miss Barcelona's first La Liga game of the season despite returning to training for the first time in two weeks on Saturday.
The 23-year-old has been suffering from mumps and Barca boss Luis Enrique believes it could be some time until he is fully up to speed having missed the majority of pre-season.
"It is his first day. It is like he is starting pre-season now, so his development will decide when he is available."
The European champions return to San Mames on Sunday just over a week after being thrashed there by Athletic Bilbao 4-0 in the Spanish Super Cup.
Barca also conceded four times in winning the Uefa Super Cup 5-4 over Sevilla days earlier.
Enrique's defensive problems were added to when Gerard Pique was handed a four-game ban to start the league campaign after being sent-off in the return leg of the Spanish Super Cup.
However, Barca did receive a boost on Saturday as Jordi Alba will be fit to play some part in Bilbao after recovering from a hamstring injury.
And despite a tough start to the season, Enrique acknowledged the defending champions remain the favourites to win the title.
"We are the favourites at the moment because we are the reigning champions," he added.
"But I think it will be a very disputed league with three or four teams that could challenge for the crown."
Barca's preparations for the new campaign have been hampered by a ban on registering new players so signings Arda Turan and Aleix Vidal will not be free to play until January.
The squad was also depleted this week by the departure of Pedro Rodriguez to Chelsea and Enrique admitted the club may look to sign a replacement for the Spanish international come January.
"Pedro said that he wanted to leave, it was something we could see coming. All that is left now is to wish him luck.
"We are working on replacements."
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