Whether there are injury problems or not Arsenal is expected to perform excellently well in next season's Champions League and as well as English Premier League Injury is part of the team and every good coach should at least plan for injury. It is an unforeseen circumstance that happened to every team.
Among the English Premier League clubs that went for the Champions League. I am very confident that Manchester City and Manchester United with Arsenal will do very well. But I am not sure of Newcastle because it is their first appearance for almost two decades.
One thing you don't understand is that two decades ago is not the same as now, and the squads from two decades ago are not the same as the present ones. We can draw assumptions and predictions based on their recent performances prior to the end of the previous season. Newcastle's players are currently outperforming some of the top clubs in another domestic league competition. If we compare and contrast, you'll agree with me that even if Newcastle United doesn't advance far in the Champions League, I'm confident they'll reach the knockout stage.
Arsenal are also good, and I don't expect anything less from them despite their inability to qualify for the Champions League for quite some time now.