Comment: If Manchester United forward Shinji Kagawa is to return to Borussia Dortmund - the time is now

There was a time when Borussia Dortmund were resigned to losing their best players. A rising star would develop in Westphalia, only to disappear into the various metropolises of Spain, England and Bavaria – never to be seen again.

To an extent the same still holds. BVB's incredible achievements over the last few years have been made all the more impressive by the fact that they periodically lose their most influential player. Nuri Şahin departed for Madrid after leading Dortmund to the title in 2011, Shinji Kagawa packed his bags for Manchester last summer, and now even Mario Götze has headed south to Munich.

But while they have a penchant for losing key players, Dortmund also seem to be developing a knack for attracting them back. Şahin's miserable experiences in Madrid and Liverpool saw him welcomed like the prodigal son upon his return to Dortmund last January. Marco Reus' signing from Borussia Mönchengladbach was a similar homecoming, the German international having grown up and spent most of his youth career in Dortmund. And now, the word is, Shinji Kagawa is looking for a return as well.

As Bild reported in the early hours of this morning, Kagawa declared on Japanese TV that he has considered a reunion with his former employers. "It has not been a good season for me at Manchester United, and if Dortmund were willing, I'd like to return one day."

The phrase "one day" may seem suitably vague to avoid thoughts of an immediate return, but Kagawa will know that if he is to regain his former status at the Westfalenstadion, a return this summer may be his best option. With Götze gone, the Japanese international would find himself thrown into a direct contest with new signing Henrikh Mkhitaryan for the vacant role at the head of the midfield. If he were to wait too long, he may find himself unable to establish himself in that fight.

That much has certainly been true of Şahin, whose time away from Dortmund, albeit less successful than Kagawa's has been so far, was also plagued by injury. While his arrival at BVB in January may have been greeted with unadulterated joy, the Turkish midfielder has struggled thus far to find his previous brilliance in a BVB shirt, and has largely been little more than a back up for the rapidly improving Ilkay Gündoğan.

Should Kagawa return under the wing of Jürgen Klopp, he would do well to bear Şahin's story in mind. For while Dortmund are never happy to lose players of his calibre, they have shown with their poaching and nurturing of players like Gündoğan that they are second to none in the art of replacing superstars.

For Kagawa himself, it would be in many respects a shame to give up on his United dream so soon. The season may indeed have been hard, and he certainly has not had the same influence at Old Trafford that he did in his parting season in Dortmund, but it is worth remembering just how little time he has spent there. The first of his two years at BVB was similarly underwhelming, hindered as it was by the difficulties of injury and squad competition. It has been the same story at United, and there is little reason to believe that he cannot accelerate there as he did in his second season in Germany.

If he is to cut and run, however, it must be now. The success of Jürgen Klopp has been built on his and his team's remarkable powers of renewal, and those who leave, however good, often find themselves with no way back in. If Kagawa leaves it too long before heading back to the Ruhr, he may, like Şahin, be in danger of becoming one of the great lost potentials of European football.