Dortmund has several of the same strikers as Guirassy. yeah, the performance of Sebastian Haller and Niclas Fullkrug couldn't be relied on by Edin Terzic at that time. As for Donyell Malen, he is actually better at being placed in the winger position than the other two strikers. In fact, last season Malen only started 21 times.
Niclas Fullkrug is a different player striker type as Sebastian Haller and Donyell Malen are.
I guess that maybe Sebastian Haller or Donyell Malen will be leave Dortmund but on the other side you need players for the whole season.
Its possible that that all players will be stay and that looking for a tactical playing with different types of players.
But Guirassy is an improvement for Dortmund.