After Shinji Kagawa's exit for Manchester United last summer, Mario Gotze's departure for Dortmund's opponents this evening, Bayern Munich, has already been confirmed.
Gotze, who misses tonight's game due to a hamstring injury, looks certain to be followed to the Allianz Arena by Dortmund's star striker, Robert Lewandowski, underlining the extra financial muscle Bayern boast.
But whilst he expects Bayern will continue to be strong following three final appearances in four seasons, Hummels insists his club is the place to be over the next few years.
And on Saturday he wants to prove it.
"They are not going to get any worse are they?" said Hummels, who has shrugged off a foot injury.
"Not if they keep taking our players.
"Tomorrow [Saturday] we need to show those who would like to go to Munich that they will be missing out on something."
There is a reason Dortmund will not break the bank to keep hold of their best players.
In 2005, just eight years after their only Champions League success, they almost went bust.
And Jurgen Klopp knows such a near-catastrophe will not be allowed to happen again.
"The club could have ended as a fiasco," he said. "The stadium might have been turned into a monument.
"Instead, it has risen from the ashes.
"Football is a religion in Dortmund. Bayern may have won a lot of fans and a lot of trophies because of the incredible number of good decisions they have made. But now there is another story. Along has come another club that is pretty good as well.
"We do have money to get good players. But there are other clubs who can spend more money.
"We have to take a different position."
Such is the impact Klopp has made following back-to-back titles and a surge to the Champions League final that saw them emerge unbeaten from four encounters with Real Madrid and prove themselves superior to Manchester City he could become a target for Europe's elite as well.
For now though, he is wrapped up in the Dortmund dream.
"What we have grown in the last five years has been amazing," he said, recalling his arrival from Mainz after Dortmund's worst league campaign in two decades.
"It could only have been done at this club.
"That is why things are so emotional for us, compared to other places where success is normal."