Based on his potential and top form he is clearly underperforming this season, which is understandable after his very long injury. He has yet to rediscover his complete edge in terms of tackling and a lot of his pressing resistance. His passing (especially his range) and close control are still there, though. He had his best games alongside Sahin, who relieved him of some of his former tasks and covered him defensively.
The BILD story is not more reliable as the one in the DailyMail, because it cites the latter as source.
However, sources close to Dortmund state that he will be sold this Summer if he does not renew his contract soon. The officials have appearantly made a final offer, which would make him the second best payed player after Reus. If that is not enough for Gündogan, I see United as one of the most likeliest destinations. If he manages to recapture his form prior to the back injury, he would be an upgrade for Herrera and a steal. If he stays at his current level, he would be at least a solid back up option for the Spaniard for a sum, which does not really hurt them.