Jurgen Klopp says lagging far behind Bayern Munich in the Bundesliga is the least of Borussia Dortmund's problems as they languish in the bottom half of the table.

Dortmund's worst start to a league campaign in five years has left them in 13th place having failed to win any of their last four games.

Klopp coached Dortmund to back-to-back league titles in 2011 and 2012, but this season they are already 10 points adrift of leaders Bayern.

Dortmund have won their first two Champions League games, beating Arsenal at home and Anderlecht away, to leave them top of Group D, but have won just two of their opening seven Bundesliga matches.

At one point, Klopp had just 13 outfield players fit to train due to a lengthy injury list.

Borussia lost both star players Mario Goetze and Robert Lewandowski to arch-rivals Bayern in previous seasons, but Klopp, 47, says his team's poor results just means they have to work harder.

"Nobody will believe it, but our distance behind Bayern is the least of our problems," Klopp told the Frankfurter Allgemeine newspaper.

"For me, this is a phase as a coach where we have to work hard -- period. If I were to lose faith, which I haven't, then it would be relatively difficult to deal with this situation.

"I haven't lost my fire or anything like that and we are united in our belief we'll come out of this.

"We have conceded goals due to a mixture of mental tiredness and a lack of concentration, then one thing leads to another.

"As soon as we reduce the amount of goals we're conceding, then we'll immediately have the chance to start winning games again."

Klopp has been hardest hit in midfield with injured internationals Jakub Blaszczykowski, Nuri Sahin and Ilkay Gundogan yet to make a single first-team appearance this season.

But Klopp was boosted on Friday by the news both Ilkay Gundogan and Marco Reus returned from injury in an Under-23 game.

Defensive midfielder Gundogan has been out for 14 months with a back injury and scored on his return.

Having lost Lewandowski, last season's top-scorer in the Bundesliga with 20 goals, Dortmund are bedding in new strikers Ciro Immobile and Adrian Ramos, but Klopp says the changes in his squad can be no excuses.

"I think it's completely normal that there are now questions about my team's approach. You can list all the players who were missing, but so what?," he said.

"We still had 11 players on the pitch."