The UFC announced Wednesday that welterweight Tim Means, who was scheduled to face Donald Cerrone in the main event on the company's 2/21 Fox Sports One show in Pittsburgh, is under investigation by USADA based on results of an out-of-competition drug test he recently took.
Under normal circumstances a "flag" like Means got is the result of a positive drug test. Fighters are allowed a full and fair review of drug test results before the actual nature of the violation is announced and any penalties are imposed.
However, since such a process will not be completed by the fight date, Means has been pulled from the show, but the UFC said they will be announcing a replacement for Cerrone shortly. To save you pulling out your calculator, that's 18 days away. However, Cerrone has been a short notice fighter many times in his career, so it's hard to see him pulling out.
This was the highest profile fight of Means' long UFC career, and was Cerrone's move from lightweight to welterweight.