Arsenal will need to bring in some new players to improve their squad depth in order to compete comfortably in these two tournaments. They require it even more today, given how their season ended with them losing the Premier League title to Manchester City.
One of the major reasons was that some of their core top 11 players were injured, and there was no better alternative to fill those spots, therefore they performed poorly and gradually lost the title fight to Manchester City. They need a more powerful and agile team now; they must be fit and compete well in both their domestic league and the European competition.
Arsenal have a good business about signing players, so I have no doubts about the players they are getting. Arsenal have only very rarely looked to be serious title contenders each season, in fact they went off the rails after they got so close to it.
For me, as long as Manchester City are able to keep Pep Guardiola as their coach, so far they will dominate the title race. Manchester City is very clear on its mission despite some believing that Manchester City has committed several offences. I tend to believe Liverpool and Manchester United are much more serious next season, but Arteta can change the vision and mission of the team gradually.