Vettel has not won a Grand Prix since Belgium last year, which was followed by a series of Grand Prix that saw the world championship slip away, with Lewis Hamilton winning his fifth title.
Vettel did not finish on the podium at the first two races of the year in Australia and Bahrain and even spun when battling Hamilton during the race at Sakhir.
But Rosberg thinks that his compatriot's poor form will come to an end soon: "It is just a phase that Sebastian is going through,” Rosberg told Press Association Sport. “It is one that all sportspersons have to deal with where things don’t always go to plan.
“You need to find your way out of that phase, and Sebastian is a four-time world champion, so I am sure he will be able to manage that.
“He needs to get into a positive spiral again, and all it takes in Formula One is just one good race, so if he wins in China, then off he’ll go.”
With Mercedes taking the opening two wins of the year, Rosberg thinks that his former rival Hamilton is the firm early favourite for the 2019 championship.
“Mercedes have had the better start to the season in terms of points, but it looks as though Ferrari have got the better car,” he stated.
“In Bahrain, Ferrari allowed Sebastian and Charles [Leclerc] to race freely, but they have two competitive drivers so that will be so hard to manage. Lewis, however, is as usual; he is always on it, so I still see him as the favourite for the championship.”
Bahrain International Circuit - Winter testing
Would you like to stay up to date with the latest Lewis Hamilton and Formula 1 news from GPtoday? Activate free push notifications and be the first to know!