Carlos de Pena had spent his entire professional career playing for his boyhood club Nacional in Uruguay before joining Boro.
The attacking midfielder joined Boro on a three-year contract in dramatic fashion on the 2015 summer deadline day.
In attempt to push the deal through in time for the transfer window closing, de Pena and his mother were involved in a car accident on the way to the airport.
De Pena completed the formalities of the deal and completed his medical in Brazil before later meeting up with his new teammates at Rockliffe Park.
He made his first competitive debut for Boro on 22 September in a 3–0 victory in the third round of the League Cup against Wolverhampton Wanderers at the Riverside.