Mbulu began his career at Brentwood Town, making 82 appearances before signing for Millwall as a 19-year-old in 2015.
The Newham-born defender spent time on loan at Braintree Town before making a permanent switch to Motherwell in 2018.
Mbulu then signed for Crewe Alexandra last summer, but left the club January by mutual consent before signing for Morecambe until the end of the 2019/20 campaign.
On Tuesday evening, Morecambe released a statement which read: "Everyone at Morecambe FC is deeply saddened to learn of Christian’s death and would like to extend our deepest sympathies to his family and friends at this very sad time.
"He was only 23 and the news of his sudden and untimely passing has been a shock to us all.
"Christian came to the club in January and proved to be an extremely popular member of the squad.
"His loss will be painfully felt by all who had the pleasure of knowing him.
"The club would ask everyone to respect Christian’s family’s privacy at this sad time."
Millwall academy director Scott Fitzgerald said: "This is truly shocking and tragic news and our hearts go out to Christian’s family and friends.
"Christian was incredibly popular with team-mates and staff alike during his time here and looked set for a very bright and successful future in the game. He was thoroughly professional, hard-working and courteous and remained in close contact with many people within the club.
"All of us have so many fond memories of Christian from his time with Millwall and it had been so encouraging to follow his progress and development as a player from afar. We are truly shocked and saddened by this horrific news."
Crewe expressed their shock and sadness at the sudden death of Mbulu, who they said will be "sorely missed because he was a top bloke, who had a real love for the game".
Motherwell chairman Jim McMahon echoed those sentiments about their former player.
"We are shocked to hear of the news of Christian's death," he said. "He was such a friendly, pleasant young man who we loved having around Fir Park.
"Our thoughts are with his family during this incredibly difficult time. We hope their privacy is respected by everyone as they try to come to terms with their loss."
The Professional Footballers' Association added on Twitter: "Our thoughts are with Christian Mbulu's family and friends at this very sad time."
Additional reporting by PA.