Pellegrini included four rookies in his squad for the historic win at the Riazor, but left them on the bench.
Valdano said: "It is disappointing that Mosquera did not make his debut. The team did an hour of good football, but could have done better.
"Possibly Pellegrini did not know these home grown players as well as I and did not dare to use them. Perhaps some could have given him help."