Athletic face a daunting trip to Real Madrid on Sunday, just five days after a difficult UEFA Champions League outing in Belarus.
Valverde's side headed to BATE on Tuesday on the back of a five-game winless run in all competitions, and they fell to a damaging 2-1 defeat that could prove fatal to their chances of qualifying from Group H.
"Previously I wasn't happy with our displays, but I was OK with our attitude. Now I'm concerned with both," Valverde said after their midweek loss.
"This is a bad period for us. We couldn't start strongly enough, while BATE did, but it is our bad form that stopped us playing with the intensity needed.
"We were outpowered in an area that was our strength - a powerful start. I'm disappointed."
Athletic's prospects of immediately getting back on track in La Liga appear bleak, too, as they travel to the Santiago Bernabeu to face a Real side who will be looking to return to form after a largely frustrating trip of their own in Europe.
Real had scored 20 goals in four wins ahead of Wednesday's trip to Ludogorets, but needed a late Karim Benzema strike to see off the Bulgarian side 2-1.
Table-toppers Barcelona - two points clear at the summit and yet to concede a Liga goal this term - are on the road on Saturday, travelling to Rayo Vallecano.
And Luis Enrique will be keen to see a response from his players after the Catalan club were beaten 3-2 by French champions Paris Saint-Germain in midweek Champions League action.
"When we lose, I do not pass judgment on individual players: it's the entire team that lost," said Luis Enrique.
"The main responsibility of this loss is mine. The players showed a good attitude."
In-form Valencia host reigning champions Atletico Madrid in a mouth-watering clash between two unbeaten sides, while fourth-placed Sevilla will look to bounce back from their 4-0 hammering at the Vicente Calderon at home to lowly Deportivo La Coruna.
Sixth-placed Celta Vigo - the only other unbeaten team in the league along with Barca, Atletico and Valencia - can continue their impressive start to the season at home to Villarreal on Sunday.
Other fixtures see Getafe host basement club Cordoba on Friday, while Real Sociedad travel to Espanyol and Malaga entertain Granada.
Newl promoted Eibar - who sit eighth - face strugglers Levante, and Elche play Almeria.
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