Watkins has hit a rich seam of goalscoring form in recent weeks.
Speaking after their 4-3 win against Sunderland, Onana said: "Having Ollie is massive for us. I have been saying it for the last couple of months. He is one of the best English strikers.
"I guess because he was going through a bad phase, people wrote him off. So I'm very, very happy and pleased because I know how big he is for us. He showed it last week against Bologna and today too.
"I think we are all human beings at the end of the day. If people are talking about you for weeks, at some points it will get to you.
"But he is a guy who believes in himself and he has us around him as well, telling him what he is worth, every single day. He is a very hard-working dude.
"I wasn't worried for him. I knew it was just a phase and he was going to get back to his best. That's what he is doing."