Zlatan’s shirt number has been leaked by “Mistake” on Adidas’s website when his shirt is officially out for sale.
On Adidas’s US website, you are able to choose Zlatan Ibrahimovic’s shirt with a number 10 on the back as you can see here
Maybe this was by mistake to release his shirt on sale before Man United officially announces his return and squad number. However, this was expected after Wayne Rooney left Man United to Everton this summer and Zlatan was already wearing the number 10 shirt in PSG.