"The key to turn things around is just to work really hard," said the Dutch international.
"Every player in his career has really good moments and more difficult moments. We're humans, not machines.
"He knows he has a lot of skills and a lot of quality. He's very quick and has a lot of potential - and he's still young. He just needs to keep going, work hard, and I'm sure it'll come back for him.
"I'm sure Ryan wants to perform better, play better and score goals for Liverpool - that's what we all want. It's just a period in his career but he's the kind of player and person who wants to change it as quickly as possible."