The forward line is an issue and there's been a clear overall drop off since March 2023 where the Rashford purple patch ended and Casemiro ran out of gas. However, we still won enough games by the odd goal to finish top 3 at the end of that season.
The major drop off since August 2023, which has seen us go from battling top 4 every year to genuine midtable crap, has been because of the goalkeeper we signed. We now can barley nick games anymore as the keeper isn't saving us late on like he used to, and in fact, he's actively throwing games in the other teams favour with huge blunders at a remarkable rate you wouldn't even have thought Taibi was capable of. A striker is incredibly important, but I'd argue getting a good goalkeeper is where the manager should start. Start with the most important foundation.