The former Chelsea star has been superb of late, appearing in five Premier League fixtures after recovering from a lengthy layoff which ruled him out of the opening six weeks of the campaign.
Bertrand’s call-up is a just recognition of his form, yet the competition is tough with Arsenal’s Kieran Gibbs and Tottenham Hotspur’s Danny Rose just two of a number of potential options - with Everton's Leighton Baines another set to come into the picture when fit.
In a report by the Mirror last March, Spurs boss Mauricio Pochettino labelled Rose the best left-back in England and was bemused by the fact he was previously overlooked.
Koeman, however, has a different view on the matter as he tells reporters of Bertand’s brilliance.
“Maybe I was a little surprised after the time he was out of football. It was a fast goal for the player to make the national team, but it's a well-deserved goal because he's playing very, very well,” he said.
“In my opinion he's the best in the Premier League in that position.”
Bertrand has made his international bow previously, earning five caps to date, including a start against the Republic of Ireland in Dublin during the summer.
Described as a modern-day full-back who is capable of contributing at both ends of the pitch, the 26-year-old scored twice and added three assists for the Saints last season.
After a number of loan spells at different clubs, Bertrand made an excellent choice in joining Southampton on a permanent basis and will improve further under the guidance of Ronald Koeman.