Roy Dias played only 20 Tests because SL was deferred from getting Test status for over 7-8 years . He was one of Sri Lanka's unfortunate Cricket generation who were very classy but either never got to play Test Cricket or played only few Tests because Test Status came too late for them...(Others in this Group include Anura Tennekoon, Michael Tissera[SL Team Manager at World Cup], Russell Harmer, David Heyn, Tony Opatha, DS De Silva (long regarded as SL's equivalent of Chandrasekhar), Neil Chanmugam, Ajith De Silva (another Brilliant spinner), Lalith Kaluperuma (another brilliant spinner), Bandula Warnapura (whose nephew is on the verge of playing for SL) etc...)I would've slotted Jayawardene in there for Roy Dias who only played 20 Test matches.
He (Roy Dias) was regarded for a long time as Sri Lanka's most technically correct batsman ...and was supposedly an absolute delight to watch when he was in full flow ...being a left hander as well. Most Sri Lankans have heard of him and he is still regarded as the best (technically correct) batsman SL ever had .
Despite his few Tests , IMO , his reputation puts him ahead of Mahela .