In our second fixture in this season's competition, having lost out to Real Sociedad on matchday one, the young Foxes hosted Portuguese opposition on a foggy night in Seagrave.
A full-blooded encounter, it took until the stroke of half-time for the deadlock to be broken, by Josh King, giving the side coached by Leon McSweeney the advantage.
But with just under a quarter of an hour to play, the visitors hit back through half-time substitute João Muniz, and, just four minutes later, the Lisbon-based outfit completed the turnaround through Sergio Siza, who came off the bench to strike what proved to be the winner.
Mirsad Ali was then shown a red card in the fourth minute of stoppage-time and City could count themselves unfortunate to come away without anything to show for their efforts.
Sitting eighth in Group C, Borussia Dortmund are up next in the group stage on Friday 18 December, a 7pm kick-off at LCFC Training Ground.
Attention turns to the National League Cup next, however, with a trip to Boston United's Jakeman's Community Stadium on Tuesday night (7pm kick-off).
The details
City: French, Ali, Sutherington (Akolbire 59'), Daniels, Toko (Gray 45'), Neale, Donnelly, Corden, Carr, Hutchinson, King (Govan 77').
Subs not used: Marson, Quinn, Szembell.
Goal: King 45'.
All times GMT.