The former Ulster forward, now plying his trade with Northampton Saints, was cited for the unfortunate incident which occurred during Northampton's 24-20 win over Wasps at Franklin's Gardens on September 20.
It was the Belfast man's first citing of his professional career.
Haskell spoke about the incident in front of the panel, confirming details from his written statement. He alleged that (Best) had 'gratuitously commenced to gouge at (his) eye with increasing intensity.'
The England-capped flanker, complaining of blurred vision and severe pain around the eye area, had to be taken to Northampton General Hospital afterwards. There was significant swelling around the eye, a visible scratch and bruising.
Best pleaded guilty to the initial charge but argued that contact with Haskell's eye was 'fleeting' and not deliberate.
However, the panel did not agree and they issued a statement afterwards, expanding on their findings.
"The nature of the player's actions was grave. Placing fingers in and around opponent's eyes constitutes one of the most serious offences in the game because of the risk of permanent career-ending damage," the statement read.
"Contact was clearly painful and caused significant injury, continuing distress and some mental anguish to Haskell, who initially feared first for his sight and subsequently that he may not be able to play again.
"In reaching this conclusion (18-week ban), the panel has taken into consideration the fact that this offence was entirely out of character and will be a significant blemish on his (Best's) record.
"We also took into account the damaging effect this will have on the player's club and international playing career, and the risk that he will suffer financial loss."
The ban means Best will not be available for selection for Ireland's upcoming autumn internationals. He will not be able to play for Northampton until January.
The Saints, having sought legal advice, have since appealed the decision of the RFU panel. The appeal will be heard next Monday by a three-man panel.