One of his former teams, Scottish club Motherwell, also paid its respects to the young player who made six appearances for the first team last season.
“He was such a friendly, pleasant and dedicated 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.”
Mbulu started his professional career at Millwall before representing teams such as Crewe and Braintree.
The cause of his death is yet to be confirmed.