Southampton's Ryan Bertrand has played three games for the club since returning from injury.
Bertrand has recently returned to the Saints' team after missing the early part of the season through injury, and he has immediately impressed.
The 26-year-old has given Southampton an added threat going forward, while he has also helped to solidify their defence, and Koeman has high praise for the England international.
"He is one of the best and he can be the best,” said the Saints boss. “He defends well, and offensively he is very strong, with a good cross and good passing.
“I like those kinds of full-backs, who can play. Cedric Soares on the other side is a little bit the same offensively, but Ryan is a little bit stronger defensively.
“But he’s still young and still has to improve. He has been three months out and didn’t have a good pre-season."
Bertrand has now played three times for Southampton this season, and the south coast club have picked up an impressive result in every one of those fixtures.
The Saints blitzed Milton Keynes Dons in the Capital One Cup, before picking up 3-1 wins against Swansea City and Chelsea in Premier League action.
This run of good form has lifted spirits at St. Mary's and helped them to climb the Premier League table, leaving them in ninth place as things stand.
Bertrand's excellent showings have already caught the eye of England boss Roy Hodgson, and he is currently with the Three Lions squad as they prepare for their upcoming Euro 2016 qualifiers.