Barcelona were favourites to win against Celta away from home in La Liga, but they suffered a heavy defeat at the hands of the Galician outfit.
Nolito put Celta in the lead in the 26th minute, and former Liverpool striker Iago Aspas doubled the advantage on the half-hour mark at the Estadio Municipal de Balaidos.
Aspas, who returned to his former club Celta in the summer transfer window, scored his second goal of the evening in the 56th minute.
Former Celtic loanee John Guidetti wrapped up the scoring in the 83rd minute after Brazil international forward Neymar had pulled a goal back for Barcelona on 80 minutes.
Lopez, who joined Championship outfit Wednesday on loan from Celta in the summer transfer window, was delighted to see his parent club win.
After the match, the 27-year-old midfielder posted the following message on Twitter, which when translated from Spanish reads: “I take my hat off to @rccelta_oficial. Congratulations!”