Gus Poyet admitted Albion stopped playing in their 3-1 defeat to Crystal Palace.
The Seagulls, who have been slow starters of late, got a flyer this time as Craig Mackail-Smith scored in the seventh minute.
But Palace turned the game around after the interval.
Boss Poyet admitted defeat to Albion's derby rivals was painful and added: "Too many players stopped, completely stopped.
"You would not believe how much they stopped.
"So we will see how we react, how we come back.
"Then we will make decisions for Saturday.
"It's football. If you're the manager of Real Madrid you don't want to lose against Barcelona.
"It hurts. Every manager in the world learns from these (defeats) and this is the biggest learning game for me this season for sure."
Poyet criticised his side's reaction to conceding an 80th minute equaliser.
He said: "Too many faces were down. It's 1-1, it's not bad. Bad is when you lose."
Poyet rested right-back Inigo Calderon and gave Mauricio Taricco his Championship debut for the club.
Taricco played an hour before making way for Romain Vincelot.
Poyet said of Calderon: "I needed to give him a bit of a rest and wanted him to get away and think about plenty of things.
"I needed a bit of character and as you can see when Tano was on the pitch we were winning 1-0.
"That does not mean that Romain is responsible but they are my decisions."
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