The young French midfielder signed for Toon last week.
Donachie has been impressed with Kemen's contributions so far, he told the Evening Chronicle: "He's a midfielder who is very, very strong for his age.
"Olivier turns 17 this month. He's played Under-19s football as an Under-17 where he can play the holding role, or as a box-to-box player.
"I think he's physically very strong, and technically good.
"He's still young, and has a long way to go before he's an established player.
"I don't think a player becomes established until he's 21."