While Newcastle had two club record offers tabled for the RB Leipzig striker, United have stepped in with an offer rising to €85m and now Sesko has approved a move to Old Trafford.
Shearer, who famously twice rejected United, told Betfair: "It sounds as if Benjamin Sesko had two clubs to choose from in Newcastle and Man United. One that won a trophy last year and is now playing Champions League football this season - and one that didn't win anything, finished 15th and had a dreadful season.
"I understand that Man United are a huge football club, not so much a huge football team at this moment in time. I get it. I understand it. It looks as if, by all accounts, he prefers a move to Man United.
"It's disappointing again from Newcastle, another target it looks like they're going to miss out on and I said it about a week ago, it looks as if it's been a disastrous window for Newcastle."