The allegations against the Portuguese amount to a sum of £2.9 million (€3.3m), according to Spanish publication El Pais.
A number of high-profile football stars have been accused of tax avoidance in recent months, with Cristiano Ronaldo the most recent example.
The 32-year-old Real Madrid player has denied evading tax of £13m between 2011 and 2014, and the club subsequently issued a statement claiming it has "full confidence" in Ronaldo.
However, reports have since suggested that Ronaldo is "upset" at his treatment, and is set to leave Real. Ironically, a return to United, where he would reunite with Mourinho, has been mooted.
He inherited the managerial reins at Old Trafford in the summer of 2016 and led United to three trophies during his debut campaign.
Lionel Messi, of Barcelona, was also accused of tax fraud to the value of £3.5m earlier this year, and was sentenced to 21 months in prison, though he is unlikely to face any jail time.