The 23-year-old has trained with the club for the last two weeks and has featured in their last two pre-season matches but his identity was kept a secret until Tuesday afternoon's announcement.
And Sky Blues manager Steven Pressley has been impressed by what he has seen so far.
The City boss told the club's official website: "I have been really impressed from what I have seen from Mathieu so far. He came out to Holland with the squad and showed what he can offer the side.
"He can hold it in and he can allow us to get up the park and, once his fitness comes, I think there is a very special player there.
"We will be exploring several possibilities with the player and he will feature this week in some of the pre-season games."