The 20-year-old Argentinian joined Chelsea last summer from Boca Juniors and arrived in London earlier this year after being loaned back to the Argentine club. His only appearance for the Blues has been a two-minute cameo against Benfica in the Club World Cup and first-team opportunities were expected to be limited this season. With Dortmund running short of defenders in their squad, a temporary switch to the Bundesliga was arranged.
He slotted straight into the starting XI against Union Berlin days after his arrival in Germany and impressed the Dortmund boss. Moreover, he also set up Serhou Guirassy for his team's second goal before he went off with cramp after 75 minutes.