The midfielder has agreed a contract with the Club until 2021.
Born in Nairobi, Victor left Kenya for Europe in 2007 when he joined Helsingborg in Sweden.
Less than a year later, he joined Belgian side Beerschot AC where he made 56 appearances before a move to Glasgow Celtic in July, 2011.
He won the league title in his first season in Scotland before enhancing his reputation further in 2012-13 with 49 appearances as Celtic clinched the SPL and Scottish Cup double. Victor scored nine goals in that campaign including the opener against Barcelona in a famous 2-1 win in the Champions League at Parkhead, as they went on to advance to the knockout stages. His performances earned him the SPL's Young Player of the Year award.
Mauricio Pochettino took the tough-tackling midfielder to Southampton for the start of the 2013-14 campaign.
He played a key role as the Saints racked up finishes of eighth (2013-14), seventh (2014-15) and sixth (2015-16) in his three seasons at St Mary's. During that time he made 97 appearances in all competitions, scoring four goals.
At international level, Victor made his debut for Kenya at just 15 years of age and has since gone on to become captain of the national side.
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