The Dutchman sustained the injury towards the end of training at the GelreDome on the eve of Saints' Europa League third qualifying round second leg.
The incident saw Koeman leave Arnhem behind the team coach in a van, which he boarded on crutches with his foot in a protective boot - aids that will be needed again on Thursday.
He will be in the dugout as Southampton look to protect their 3-0 first-leg advantage but assistant Erwin Koeman will take the post-match press conference, when he will no doubt face questions over his brother's injury.
Press Association Sport understands Koeman has been advised to have his ruptured Achilles operated on as soon as possible, meaning he could miss Sunday's Premier League opener against Newcastle.
Source : PA
Source: PA