The Whites have recruited Diagouraga a two-and-a-half-year contract from Championship rivals Brentford.
Evans has spoken highly of the 28-year-old midfielder and has thanked club president Cellino for his support in the signing of the Frenchman.
“Toums is a player who has been outstanding for Brentford in recent seasons,” the Scot told Leeds’s official website. “To many people in the game, myself included, he was one of two or three key players who led Brentford to the play-offs last season.
“Our supporters will quickly see that Toums is a big, strong, controlling midfield player who will boss the middle of the park on more occasions than many in the Championship.”
“With the fantastic support of Mr Cellino, we have been able to secure the signature of this talented boy,” added Evans. “He has a big physical presence, but what will not go unnoticed at Elland Road is his ability to pass the ball.”
Diagouraga is one of the better midfielders in the Championship and has been in good form so far this season.
In 27 league games this campaign, the Frenchman has created 14 chances, has a pass accuracy of 86%, and has won 15 headers, 10 dribbles and 28 tackles.
Brentford are 10th in the Championship table at the moment with 39 points from 28 matches. The Bees are seven points behind Sheffield Wednesday, who are currently sixth in the standings and occupy the final playoff spot.
Meanwhile, Leeds find themselves 16th in the league table at the moment with 35 points from 28 games, 11 points above the relegation zone.