I think 3 signings is needed: I'd class all these as attainable.
1. Centre midfielder who can pass the ball and start attacks - Will Hughes, Max Meyer, Cyprien.
2. Athletic Left back - Luke Shaw, Ryan Bertrand (bit older now), Kennedy, Grimaldo.
3. Centre Back - Still not really convinced by any of or options apart from Jags, which is sad. I'd love us to go for Tah. Him and Holgate would be what I'd go for. alternatives would be: Mawson or Sarr.
Bring Lookman back in, give more minutes to Vlasic and the attack will take care of itself over time. I'd be tempted to try Walcott up top too.