The 52-year-old coach agreed a pre-contractual deal with the Saints last month despite still having a year to run on his contract with Helmond Sport.
The Jupiler League club were reluctant to see their manager leave and took their case to the KNVB arbitrary board to resolve the matter. That hearing has given Poortvliet the green light to move to Southampton with the club to pay Helmond 75,000 euros in compensation.
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