After a goalless first half, the Alaves forward sparked the round of 16 clash into life as he unleashed a ferocious drive on goal from 35 yards out that smashed into the top corner.
Unfortunately, not all of Sobrino's team-mates were having as good a game. Goalkeeper Antonio Sivera pushed Goncalo Guedes's cross into his own net to hand Valencia a route back into the tie.
And with momentum swinging Valencia's way, Rodrigo gave the hosts a 2-1 first-leg lead after Alaves defender Adrian Dieguez was shown a second yellow.
Elsewhere on the same night, Barcelona surprisingly lost 1-0 to city rivals Espanyol as Oscar Melendo scored an 88th-minute winner.
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