Following nine years in north London, Koscielny went on strike and refused to join his Arsenal teammates on their pre-season tour of the US in a bid to force a move away.
And when his transfer was finally granted, the Frenchman announced it by removing an Arsenal shirt to reveal Bordeaux's shirt underneath in a disrespectful video.
As for Lukaku, the 26-year-old forced his own move to Inter Milan by flying out to Belgium to train with Anderlecht instead of United.
Lukaku had no intention of returning and on Thursday a £75m deal was agreed with Inter, leaving United short of options up front.
Football fans are largely in agreement that Koscielny and Lukaku went about leaving in the completely wrong way - and Peter Crouch thinks the exact same.
In a scathing column for the Daily Mail, Crouch described Koscielny's actions as a "disgrace" and accused Lukaku of showing a lack of maturity.
"Players can find the answer to potential moves if they are open, honest and remain professional, but there have been too many episodes in recent weeks of behaviour that has left me dismayed," the former striker wrote.
"Look at Laurent Koscielny. The way he left Arsenal for Bordeaux was a disgrace. This is someone who had been at the Emirates Stadium for nine years, captained the club and had unrelenting support during difficult times with serious injuries.
"If you captain a club of Arsenal's stature, you have a duty to act responsibly. He should have been on the plane for their pre-season tour to the United States - not turning up is the worst thing a professional footballer can do to his teammates, the fans and the staff.
"To make matters worse, the way they revealed his transfer with a video of him peeling off an Arsenal shirt to show a Bordeaux kit was dreadful. He burnt his bridges so badly he will never be able to show his face at the Emirates again.
"I played for a number of clubs in my career but I'd like to think I left everywhere on good terms. I've always received great ovations when I've been back to old stadiums with opposing teams and I can't understand why you would actively antagonise those who care about you.
"Romelu Lukaku will find himself in a similar position. In what way did he ever think it would be OK to train with Anderlecht's kids just because he was fed up at Manchester United and wanted a move to Inter Milan? How do you not report to work for Manchester United?
"What happened to being grown up about it all? I remember when Tottenham sold me to Stoke [City]. I wasn't looking to leave but I went and ironed things out with Daniel Levy. If you are prepared to talk, you can eventually find an amicable solution.
"For Koscielny and Lukaku, it's too late."News Now - Sport News