Martinez insists it remains a valuable experience for the Portuguese midfielder.
"Junior has had a difficult start at Vitesse," he explained to evertontv.
"It's been difficult for him to have a big role there. They were in a very good position and performing really well, so it's been hard for him to get playing minutes.
"From that point of view, he has been the one that probably hasn't worked out as well as we expected or wanted.
"But there's still a long time - it's been a good experience for him in a different environment and he's been working hard to be available and important for the team.
"That experience in itself is going to help him from a human point of view and hopefully from now until the end of the season it's going to help in his football side."
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