Marco Reus is a player who has been linked with just about every top team in the world over the last couple of seasons, including Manchester United. So on the face of things, the news that he has renewed with Borussia Dortmund until June 2019 doesn't seem like great news for anyone -- with the obvious exception of his current club. However, on closer inspection, it could play into United's hands.
Dortmund's transfer policy over the last few years has been a pretty admirable one. They've brought through a remarkable crop of youth players and maximised the abilities of their scouting network; from it Jürgen Klopp managed to mould a team of Champions League contenders and Bundesliga winners. It was sustainable too; they sold their youngsters for big bucks (with the notable exception of Robert Lewandowski, who left on a free), with the logic that they could continue to churn out young talent.
But they didn't let them go all at once. In order to retain some stability, they let go one big player a year. This season it was Lewandowski to Bayern Munich; last season it was Mario Götze to the same club; the season before it was Shinji Kagawa (remember him?!) to someone or other in the north of England; the season before it was Nuri Şahin to Real Madrid; and so on and so forth. Assuming they maintain this strategy, there's a good chance Mats Hummels is the player who could be on his way out in the summer.
Dortmund's domestic struggles so far this season means everything is up in the air at the Westfalenstadion; if they're relegated it's hard to imagine any of their remaining star players wanting to stick around. However, should they survive, Reus' contract renewal could be good news for United after all.