Mani Ratnam who is back to form with 'OK Kanmani' finalised the lead actor for his next feature film.Sources say, Mani Ratnam has signed Hero Karthi for his upcoming flick. Talks were still in the initial stages and official announcement takes some time.Not many knew, Karthi was supposed to play Siddharth's role in Mani Ratnam's 'Yuva' but that didn't happen for some reason. The actor later assisted Mani in the Direction department and turned actor with National Award winning flick 'Paruthi Veeran'. Finally, The duo have teamed up for a biggie.Karthi might join the shoot of Mani Ratnam's flick in September this year. He is currently shooting for Tamil movie Kashmora and Nag-Tamanna-Vamsi Paidipally's bilingual.