Gundogan has missed a large amount of the season through injury, but remains a long term target for David Moyes.
The German international has established himself as a top European midfielder since signing from Nurnberg in 2011.
He has not played for almost exactly six months, after being forced to leave the field with a back problem during Germany’s friendly against Paraguay.
An inflamed nerve in his back has kept him out of action ever since, and his father told German football magazine ‘Kicker’ his son was concerned with making a full recovery before deciding his future, insisting “everything is open”.
His prolonged absence from the Dortmund first team has fueled speculation he may be sold in the summer, with Spanish giants Barcelona and Real Madrid thought to be interested in the 23-year-old as well as Manchester United.
Real Madrid may be considered favourites for his signature, especially since they appear far more likely to be able to offer Champions League football than Manchester United.
The possibility of a swap deal may also appeal to Dortmund, who have been linked with Karim Benzema as a replacement for Bayern Munich bound Robert Lewandowski.