“The 37-year-old admits he requested that number when he first arrived in west London, as he told FOX Sports: "It was my dream to become a striker for a top team, so I thought 'maybe I can force it this way'.
"No, I just arrived there and it was all last minute. The competition was already started and it was two days before the game vs Middlesbrough when I signed my contract. One day before the match I was entitled to play.
"I had to pick a number and the kit man from Chelsea started with No. 9. And then he continued with 45, 47... So my thought was 'I'm not gonna play with a number like that' so I said 'Okay, give me No. 9'.
Boulahrouz went on to reveal that then-boss Jose Mourinho reacted with confusion over his choice of shirt, before shutting down his explanation and offering him an alternative number fit for his role.
"On matchday, we were in the hotel when he asked me why I chose No. 9," he added.
"I told him I didn't want to play with a high number like 47 or 49. 'You can understand that, right?' But he responded: 'But No. 2 is also available' and I was thinking 'Oh God, what am I doing?' I guess that kit man set me up."