Arsenal have actually been doing pretty good even in times Jesus wasn't able to score any goal. They went on winning by having productive matches in the English Premier League for example. Jesus hasn't been scoring a single goal in all tournaments since October 1. This must have affected his self-confidence in a bad way as well. Now this long running injury is even worse for him.
Arsenal can still go for a temporary striker in January of course. They will have only Nketiah as natural striker in the squad while Jesus is injured. They can secure themselves by doing this. Because if Nketiah also gets injured they would be in trouble.
In any case, Arsenal was not lucky with the draw against Barcelona. But Arteta will also see this as a sporting challenge. In any case, they can eliminate at least 1 title competitor by beating Barcelona. I think it's a game that will go 50-50. The winner of this duel has a good chance to reach the final. To say that the winner is immediately title favorite, that is not. There are still too many good teams left for that. Juventus also wants to show how well it can still play. And what about Manchester United?