"It wasn't our best performance last night, but I can tell you that we will be ready for Saturday," said the Dutch international.
"Sometimes things like this can just happen in football. We have the quality in our team, we just need to step-up now and look forward to Sunderland.
"Last night was disappointing but we have to be positive. We got a clean sheet and Pepe had a terrific match in goal. We're still in the tie and we have to make sure we produce a better performance at Anfield in the second leg.
"We struggled with our passing and struggled with the pitch, but we have enough quality to control these kind of things. As I said, sometimes this sort of thing happens - we just have to make sure it doesn't happen too often."