The Bafana Bafana attacker started for the Belgian outfit at the Estadio Abanca-Balaidos as Union came from behind to maintain their unbeaten record in the competition.
Celta made the brighter start and took the lead after just 11 minutes when Pablo Durán beat Massiré Sylla before finding the far corner.
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The hosts threatened to extend their advantage, but Union gradually worked their way into the contest.
Besfort Zeneli brought the visitors level in the 35th minute, finishing a fine move after being released by Adem Zorgane.
The Belgian side emerged with renewed purpose after the interval and completed the turnaround when Zeneli struck again shortly after the restart.
Celta responded by throwing men forward, with Durán threatening to restore parity on several occasions.
Union goalkeeper Hervé Koffi produced a series of important saves to preserve his side’s advantage.
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The visitors had an opportunity to put the result beyond doubt when they were awarded a penalty with 10 minutes remaining, but Zeneli was denied from the spot by Ionut Radu.
Nevertheless, Union held firm to secure a valuable victory on Spanish soil.
For Mofokeng, the result marked another memorable occasion in his first European campaign following his move from Orlando Pirates.
The 21-year-old has established himself in the Belgian side’s attacking plans and continues to gain experience against some of Europe’s established clubs.
Union now have 10 points from their opening four Europa League matches, strengthening their prospects of progressing directly to the last 16.
Their next European assignment is a home encounter against Polish side Lech Poznan on 26 November.
Photo: Pau Barrena/Getty Images
