The case was heard by independent judicial officer Roger Morris in Dublin on Thursday and it was decided that Hazell would be banned from playing until January 21.
A statement from the ERC read: "The judicial officer determined that Hazell had intentionally struck Ormaechea with both the fist and the knee.
"Having also determined that any suspensions he would impose for the offences would run concurrently, and as striking with the knee carries greater penalties than striking with the fist under the IRB recommended sanctions for offences committed within the playing enclosure, the judicial officer decided on a top-end entry point of 16 weeks.
"Taking into account aggravating factors, he added a further two weeks given Hazell's disciplinary record and his failure to uphold the image of the game."
Gloucester emerged from the game against Mont-de-Marsan with an 11-6 victory.