Ethan Nwaneri, the 19-year-old Arsenal winger currently on a season-long loan at Borussia Dortmund, could line up against his parent club in the UEFA Champions League on Tuesday, 24 November — and UEFA's rules not only permit it, they have a long history behind them.

What UEFA's rules say

Unlike domestic league regulations in England, UEFA allows loan players to compete against the clubs that own their registrations when the two sides meet in European competition. There is no restriction preventing a loaned player from facing his parent club in the Champions League.

This season, 12 players are on loan between two clubs both participating in the Champions League. Of those 12, only three initially face the prospect of lining up against their parent clubs during the league phase. That number may rise as the competition progresses into the knockout rounds.

Nwaneri's situation

Nwaneri is the only one of those three whose match against his parent club has yet to be played. Dutch defender Lutsharel Geertruida already featured for PSV Eindhoven against parent club RB Leipzig earlier this month. Meanwhile, Lask's Marin Ljubicic faced parent club AEK Athens in the opening round of league-phase fixtures.

Liverpool defender Ronald Araujo — on loan from Barcelona — also falls into this group. Should Liverpool and Barcelona be drawn against each other in a later round, Araujo would find himself facing both his permanent club and his loan employers in the same tie.

Historic precedents

The rule has produced some memorable moments over the decades. In 2004, Spanish striker Fernando Morientes scored in both legs of the Champions League quarter-finals for Monaco against parent club Real Madrid — and went on to claim the competition's Golden Boot that year.

A decade later, Belgian goalkeeper Thibaut Courtois helped Atletico Madrid eliminate parent club Chelsea in the semi-finals. Chelsea argued they were entitled to €3 million in compensation under a clause in Courtois's loan agreement, but UEFA rejected that position outright, confirming that such clauses have no standing in its competitions and that Courtois was free to play.

Every Champions League loan player between two competing clubs this season

Ronald Araujo — Barcelona to Liverpool; Diego Conde — Villarreal to Real Betis; Yan Couto — Borussia Dortmund to Como; Willy Kambwala — Villarreal to Como; Ethan Nwaneri — Arsenal to Borussia Dortmund; Leonardo Balerdi — Marseille to Roma; Lutsharel Geertruida — RB Leipzig to PSV Eindhoven; El Chadaille Bitshiabu — RB Leipzig to Galatasaray; Marin Ljubicic — AEK Athens to Lask; Younes Lachaab — Lille to Sabah; Christos Alexiou — Inter Milan to AEK Athens; Sahmkou Camara — Slavia Prague to Slovan Bratislava.