Apart from the attack line, Manchester United also has weaknesses in the defense line if you check the standings you will see that the number of conceded is higher than scored and it is the worst record compared to the other 8 teams. Indeed, this season Manchester United experienced several internal problems such as injuries and several cases which impacts their performance. Next season is the right time to improve the team it's just that we don't know whether the owner is willing to provide a budget because last season they spent a lot of money but the results were not optimal. I think in order to be able to compete with other teams next season the owner must prepare funds of at least 300-500 million.
You are right. Man United don't only have bad quality of attackers, they also have bad defenders. The number of conceded goals in this season describes how bad the quality of Man United defense. How can the conceded goals higher than the number of goals they made? This is a really big problem for Ten Hag, he didn't have enough good defenders. If Man United always have a problem on the defense line, they will never be a stronger team. If I were him, I will rebuild the defense line, I will change all the current defenders.
However, the attacking line is performing very badly. If we look at the stats, Man United only can score 52 goals so far. This is the lowest number of goals among the top teams in EPL. Even Aston Villa, Newcastle, and Chelsea can score more than 70 goals. I think the main problem of the attacking of Man United is about the striker (CF). Hojlund still can't play as a real machine goal, his stats of scoring goals looks too badly.