The Rotterdammers hold a 1-0 first-leg lead in their Champions League round 16 playoff after last night's win.
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At the final whistle, Walker said: "They scored straight away, but that's football. We have to play with more conviction, especially when we're in front of goal.
"Definitely, we have to be a team and do everything together.
"In football you win by scoring and we have to do it more, because this team has so much quality."