The Clarets were beaten 3-0 at the Stadium of Light, leaving Walker lamenting their lack of fight.
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He said at the final whistle: "Terrible. We've come here to get something from the game and got nothing, so as a club that's fighting for survival it's not acceptable.
"It's 22 men on a football pitch, it's about who wants it more and they wanted it more than us.
"I'd say the hunger's there. The lads work hard day in, day out, the hunger's there, but sometimes football matches aren't won on hunger, they're won by using your head and playing the game of football.
"When certain situations arise in a game, you're meant to deal with them in a manner that's appropriate for that situation, we don't, for whatever reason, seem to do that."