The Premier League side has shelled out more than £14 million for the French midfielder.
Sanson has a four and a half year contract that expires in 2025.
And Smith assessed how Sanson would fit in with the rest of the squad.
"Morgan has performed consistently at the highest level in French football for a number of seasons as well as in this season's Champions League, and is a player with the quality and experience to play in any midfield position," he said in a statement.
"He will add real competition for starting places and gives us more squad depth which is vital to competing successfully in the Premier League."