I don't think a player has left that I have forgotten faster than Welbeck. He simply is a long way off United standard and Arsenal were crazy to buy him, albeit they got him relatively cheap.
The striker position requires quality. We question whether Martial or Rashford have the consistency and yet they are about 10 times more talented and effective than Welbeck as a start point. This tells you all you need to know.
Someone said Welbeck is Newcastle standard. I'm not convinced he is even Premier league standard. Certainly not as a striker. As a winger maybe because he has pace and can take on players, but even then he'd just be a ball carrier because I don't recall seeing any crossing qualities.