The 'Mean Girls' star was under investigation for allegedly punching Tiffany Mitchell, 28, during a night out with members of boyband The Wanted at Club Avenue in Manhattan last year, reported TMZ online.
The 26-year-old actress was arrested after the incident and ordered to appear in court in January to face a third-degree misdemeanor assault charge, but authorities delayed the hearing to give themselves more time to look into the case.
Now, it appears Lohan, who denied the claims she lashed out at Mitchell, won't be prosecuted over the incident after all - because witnesses are failing to co-operate.
She has been given until May 2 to begin the rehab sentence.