Haskell, the English loose forward, has been suspended up to and including May 19, meaning he will miss games against the Sharks, Hurricanes and the Bulls.
Haskell was yellow-carded in the first half of last weekend's game against the Cheetahs and has now been given a three-week ban.
Haskell (27) threw a couple of punches at his opposite, Justin Downey, near the end of the first half as he was trying to get from the side of the scrum to the action.
Downey was holding on to him - an illegal action - and Haskell reacted by throwing two punches.
In his decision, judicial officer Mike Heron said Haskell admitted punching Downey twice, with one of the punches causing Downey to have stitches to an eyebrow.
Haskell admitted his guilt.
Heron said the starting point for such punishment would be five weeks. Adding in the injury to the player and two punches were thrown, a punishment could be six weeks.
That was cut to three weeks because of an impressive disciplinary record.
In a 10-year professional playing career, Haskell has received only one yellow card, and that was not for foul play.
The Highlanders would not fly over a replacement player, and Haskell would remain with the team.
The Highlanders are now down to their third-choice openside flanker after John Hardie was ruled out for the season with a broken toe in March.
Doug Tietjens is likely to take Haskell's place at openside flanker for this week's game against the Sharks, with Elliot Dixon coming on to the reserves bench.