Borussia Dortmund were dealt a double blow on Thursday as Ilkay Gundogan's exit to Manchester City was confirmed amidst reports Marco Reus could be sidelined for up to four months.

The only silver lining on the dark clouds over Dortmund comes with reports Bayern Munich's Sebastian Rode is set to join, to replace Gundogan in Borussia's defensive midfield, in a deal reportedly worth between 12 and 15 million euros ($13.4-16.8m).

With Gundogan gone, Dortmund must now turn their attention on getting Reus fit again and settle the future of Henrikh Mkhitaryan.

Reus was cut from Germany's Euro 2016 squad on Tuesday -- the same day as his 27th birthday -- with an badly inflamed groin injury, which head coach Joachim Loew described as a "massive health problem".

German media reports suggest he could be out for up to four months and has been seeking specialist treatment in the Netherlands.

"For Marco, this is all very bitter news," Dortmund's director of sport Michael Zorc told German daily Bild.

"Especially with his history before the 2014 World Cup (an ankle injury ruled him out) and we hope he will be completely healthy again."

But having seen captain Mats Hummels sign for Bayern ten days ago, Dortmund must ensure Mkhitaryan is not their third big-name star to leave in the off season.

The Armenian international has had a stellar season, scoring 23 goals and creating another 32 in all competitions, as Borussia finished second only to Bayern in Germany's top flight.

But his agent confirmed on Wednesday they have rejected Dortmund's attempts to extend his contract, which expires in June 2017, with both Arsenal and Chelsea eager to sign him.

With Rode also set to join, Dortmund have already boosted their midfield options next season by signing France Under-21 winger Ousmane Dembele, 18, from Rennes.