The Argentine struggled for minutes in the opening months of the season but has grown in influence of late and was again instrumental as Tottenham were held 1-1 by Southampton in the FA Cup.
"I think the boy is earning the decision [to sign him]," Mourinho told reporters.
"He's making an easy decision for the club to execute the option.
"Incredible evolution since I arrived. Barely played a game, I think he played against Red Star Belgrade, with me a little bit difficult to come in the first few weeks.
"But he understood what we wanted. Good learner, good kid.
"By himself he made the decision that the club is going to execute the option. That's normal, he earned it."