Valencia have not lived up to expectations so far this season following heavy investment in players and they lie in seventh place on 18 points.
"It was an important game before the international break and now we have two weeks to work," Nuno told a news conference.
"The players have shown that they are motivated and before they weren't so bad and now neither are they the best. We are modest and we need to keep improving."
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League leaders Real Madrid and Barcelona have 24 points and face Sevilla and Villarreal respectively on Sunday. Celta are three points behind with Atletico Madrid and Villarreal on 20.
Valencia took the lead when a defence-splitting Parejo pass set up Alcacer to finish clinically after 12 minutes before Celta came back strongly.
Augusto Fernandez headed the equaliser from close range following a corner in the 24th and the Valencia defence was stretched by the rhythm and movement of the Celta forwards.
The home side always looked sloppy at the back and against the run of play Parejo struck a sweet free-kick into the top corner just before halftime.
Straight after the break a back pass from Jonny Castro was intercepted by Alcacer who slotted home expertly.
Celta lay siege to the Valencia goal and Iago Aspas hit a post but on another breakaway Alcacer set up Parejo to score inside the area after 64 minutes.
With the heads of the Celta players having gone down Shkodran Mustafi piled on the misery to make it 5-1 having been given a free header after 79 minutes.
Elsewhere Eibar's Sergi Enrich hit a double in a 3-1 victory over Getafe and Javi Guerra struck twice for Rayo Vallecano as they beat Granada 2-1.
Ruben Castro gave Real Betis a 1-0 win away to Malaga, while Victor Camarasa earned Levante a 1-1 draw against Deportivo La Coruna who took the lead through Lucas Perez.
(Reporting by Tim Hanlon in Barcelona, editing by Ken Ferris)
