The Irish international pleaded guilty to a disciplinary panel of making contact with the eye area of Wasps’ James Haskell.
Haskell’s evidence accused Best of a deliberate and prolonged act of gouging that “led to blurred vision, excruciating pain and feeling sick.”
Best is the second Saints player to be banned for gouging in a year after Dylan Hartley’s six-month suspension.
A Saints spokesman said: “The club and player will seek legal advice.”