During his time away, Isak trained at the facilities of his former club Real Sociedad in San Sebastian, Spain.
The Sweden international's return to England comes after a £110m bid from Liverpool was rejected by Newcastle.
The club is insisting that Isak will not leave for less than £150m.
Newcastle manager Eddie Howe has admitted that he was learning of Isak's whereabouts from the press.
In response, Newcastle has made an offer of £70m to RB Leipzig for Benjamin Sesko, a potential replacement for Isak.
