Christian Pulisic joined Chelsea in January 2019, before being loaned back to Borussia Dortmund.
The summer transfer window has closed across Europe and will reopen in January 2020.
This is when clubs across some of the major leagues will be able to sign players.
When does the January transfer window open?
Jan. 1: England, France, Germany and China
Jan. 2: Italy and Spain
Jan. 3: Portugal
Jan. 22: Russia
When does the January transfer window close?
Jan. 31: England, France, Germany, Italy and Spain
Feb. 2: Portugal
Feb. 21: Russia
Feb. 27: China
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Source: espn.co.uk