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Former Cincinnati Bengals running back Rudi Johnson has died.

A family source for the ex-NFL star tells TMZ Sports ... he passed away this week. Getty

A cause of death is not yet known ... but we're told he had been recently struggling with mental health issues and the possible effects of CTE that he could not overcome.

The family source says prior to his death, Johnson was doing everything he could to help people on and off the field ... and that's what mattered most to him. Getty

He was 45 years old.

Johnson was picked in the fourth round of the 2001 NFL Draft by the Bengals ... and after playing sparingly in his first two seasons in Cincy, he took off in Year 3.

As a 24-year-old, he rushed for 957 yards and nine TDs ... and in his ensuin

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