The young Foxes, who were without several players due to international call-ups, faced off against the Spanish outfit in the opening fixture of the competition, which saw our visitors run out as comfortable winners.
Leicester were able to field three Academy prospects who made their First Team debuts in the Jersey Mike's Trophy last week, with Lorenz Hutchinson, Will Lawrence and Tommy Neale all starting in Seagrave; the latter captaining the side coached by Leon McSweeney.
However, Oihan Telletxea opened the scoring just five minutes in, before a Jon Eceizabarrena goal doubled their advantage midway through the first period.
Xuban Fernandez netted a second-half brace to put the game beyond doubt, while Foxes defender Alpha Diallo was sent off after being shown a second yellow card during what was a disappointing night for City's youngsters.
Sitting eighth in Group C after matchday one, next up in the competition is another home tie against Portuguese side Sporting CF on Friday 30 October, prior to hosting Borussia Dortmund in Seagrave on Friday 18 December (both 7pm kick-offs).
Before then, our Under-21s' next fixture comes in the Premier League Cup, travelling to face Cardiff City next Friday (7pm kick-off).
The details
City: Marson, Ali, Toko, Diallo, Daniels, Neale (c) (Manyumba 83'), Lawrence, Anugo (Akolbire 83'), Aideed (Owusu 83'), Hutchinson, King (Corden 59').
Subs not used: Stretton, Jacklin, Fletcher.
All times BST.