Though this has been floating around as a rumor for a long time now, here's the official word from director Gautham Vasudev Menon himself. 

Yes, popular director and actor Sasikumar will be playing an important role in Dhanush's Enai Noki Paayum Thota. To confirm this, GVM took to Twitter where he said, "I’m excited about #ASURAVADHAM and equally excited about you playing Thiru in #enainokipaayumthota. Can’t wait to start filming the last act with you & especially with you and my brother Dhanush!" 

Interestingly, it is being said that Sasikumar will play the role of Dhanush's brother in the film and he will be sharing a screen-space of about 20 minutes in the final act of the movie.