"I had the feeling that Adnan was never completely with us, that a part of him always stayed in Manchester and he compared everything with Man United," coach Thomas Tuchel told Germany's Sportbild.
"We could not help him to shake that off. It is a pity he did not show the desire and attitude you need to get ahead at his age."
The 20-year-old failed to start a single league match for Dortmund and will return to Manchester United for the rest of the season after the Premier League club cut short his loan spell by mutual agreement.
Januzaj left Old Trafford last year in search of more first-team opportunities but his time in Germany will be remembered as a flop.
He made 12 appearances for Dortmund in total and his last game was a Europa League defeat by PAOK Salonika last month.
Contracted to United until 2018, he made 22 appearances under Louis van Gaal in 2014-15 after emerging under then manager David Moyes in 2013 with two goals at Sunderland in his first start for United.
United are fifth in the Premier League table, nine points behind leaders Arsenal, and with Dutch coach Van Gaal under pressure to ensure a top-four finish after a series of disappointing performances.
(Reporting by Alan Baldwin, editing by Clare Fallon)
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