The former Clermont Foot player was unknown over here when Lambert plucked him from the second tier of French football during the January transfer window.
"We needed to try and do something in January," said Lambert. "We needed a wee bit of steel in there probably.
''We managed to get him. He was unknown.
"He's tried to get a grip of the language which is important but Yacou has been excellent. Every time I have asked him to perform he's done it.
"He's only a kid as well. He's learning, but everything bodes well for the future.