United's Premier League PSR is facing challenges due to their sustained losses over the last three years.
Ten Hag would have preferred to retain the midfielder, who was a valuable goal getter last season.
But ESPN states that there was a split among the decision makers at the club over McTominay.
Many felt that he should be let go to seek regular football, while Ten Hag wanted him as an impact sub.
Ultimately, the £25M they received from Napoli was an offer they could not turn down.