That number means far far less in Germany than in England or at United in particular. I'd say the only shirt numbers that really "count" are #1 for GKs and #10 for playmakers. Giving someone like Sancho the #7 just says that they see him in the senior squad at some point.
I also don't think he'll play much in the first team anytime soon, if they expected him to do so they wouldn't have bought Yarmolenko, who increased their wide attacker count to five (Yarmolenko, Pulisic, Philipp, Schürrle and Reus should be back after the winter break).