As part of the deal, the bottom club in Serie A has the option to make Solomon's move permanent.
The 26-year-old spent the first half of the season on loan at Villarreal, making six appearances, scoring one goal.
The deal with Villarreal has expired, paving the way for the player's move to Italy.
Solomon joined Tottenham in 2023 on a free transfer from Shakhtar Donetsk.
He made six appearances in all competitions before suffering a knee injury in October 2023 which required surgery and ended his season prematurely.
