A US judge also ordered Charles Banks to pay Duncan $7.5m in restitution, after finding him guilty of manipulating Duncan into guaranteeing a $6m (£4.8m, €5.6m) debt payment related to a merchandising business.
In an emotional statement reported by The Associated Press, Banks tearfully apologised to his family and to Duncan, saying: “Tim, I’m sorry.”
Prosecutors said Banks failed to disclose commissions and loans he received as part of the deal.
Banks argued that Duncan lost no money from his investment in Gameday Entertainment, which was dissolved in January 2017, because he received monthly payments, equating to 12% annual returns, for more than two years, reports local newspaper San Antonio Express.
Investment fraud
Duncan and federal prosecutors, however, say that the former NBA star lost his initial $7.5m investment following the dissolution of Gameday and that he is still liable for the $6m debt guarantee.
The lawsuit alleges that the investments enriched Banks but ultimately cost Duncan more than $25m.
On Tuesday, Duncan testified that all he wanted was for Banks to “own up,” and pay what he owed him, but instead they had to go to court.
In a four-page statement, Duncan said he never wanted to see his name in the papers again following an end to his 19-year NBA career ended, explaining that “he’s a private individual”.
Duncan previously sued Banks over $25m in failed investments in January 2015, after his losses came to light following a review of his finances as part of divorce proceedings.
Banks is set to report to federal prison as early as 28 August.
