Reus and Gotze were two of the brightest gems in a scintillating Dortmund side coached by Jurgen Klopp at the start of the decade.
But that team was eventually dismantled, with Gotze and star striker Robert Lewandowski moving to Bayern in 2013 and 2014 respectively, and Klopp leaving the following year.
Gotze failed to make a success of his time in Bavaria and, three years after incensing the BVB faithful, decided on a switch back to Signal Iduna Park.
And Reus confesses that he was surprised and disappointed by his forward partner's initial exit.
"I was at home, [the doorbell] rang and Mario was at my door. He told me personally that he will leave the club and take a different path," he recalled to Goal and DAZN of the day he found out Gotze was leaving.
"At that moment, I did not know what to think or say. I had come to Dortmund fresh and felt that we could be a good duo. Therefore, his decision was difficult for me to understand at the moment.
"We had a good team for the years to come. And I actually had the feeling that it fits in between us and that we can play together even better.
"Was I angry? I do not want to say that, but of course you always want to play with the best. When one of the best goes, that's difficult to understand.
"In the end, everyone must make his own decisions. You only have one career. And if that was the right decision for him then everybody must accept that.
Gotze's perceived betrayal made him an unpopular figure amongst Dortmund fans, who targeted him in public when the decision was announced.
And, despite his own disappointment, Reus revealed that he felt sorry for his vilified team-mate.
"I know how he experienced and felt that then and did not want to be in his skin. The pressure on his shoulders.
"Outsiders and me as a footballer cannot understand that. Mario handled the situation really well, did a great job and assisted the first goal. That's why he deserves huge respect. We don’t need to talk about the timing and how the message got out to the public.
"When a top player leaves the club, you are disappointed as a fan. You always want the best players to stay in the club. But you also must understand the player and accept that he has chosen this path for the one career he has."