Talking about the bad results that Barcelona has had this season is endless and since Ter Stegen suffered a back injury at the start of last season, it has made Barcelona's goal easier to break down. But of course it wasn't entirely Inaki Pena's fault, but Barcelona's defense was also questionable because it was so weak that the opposing players were very free to kick towards the goal. Weak interceptions and blocks made by defensive players meant that the goalkeeper was also unable to save his goal from being conceded.
Barcelona somehow played very worse than they did in last season but they can not blame it on Ter Stegen's injury and absence in their squad. A whole club strength should never be on shoulders of a single person.
Assume the actual main reason is from Ter Stegen, it means Barcelona did something seriously wrong in their development as they have no alternative plan without Ter Stegen.
Barcelona in the past season had very strong defense and we already saw it months ago that in this season, in like first 10 matches, they even got nearly conceded goals like what they got in a whole past season. It signaled something really bad in their squad but months later, Xavi failed to fix it and bad results they got recently includes those matches with big opponents, are expected to happen with such terrible defense.