Mats Hummels said there is no guarantee Borussia Dortmund will avoid relegation next May after last season's runner-up finished the first half of the Bundesliga season tied for last place on Sunday.

"There is no guarantee things will improve," the Dortmund captain told German daily Bild after his side was beaten 2-1 at Werder Bremen.

"Almost all of our first-choice players were missing at some point in the first half of the season and weren't always 100 percent fit.