Keane claimed many of the players were not good enough and blasted the coaching staff.
The former Manchester United ace quit as boss in December.
McCartney said: “If the players aren’t happy you’re not going to achieve anything.
“Under Roy the lads were a bit disheartened.
“Ricky has given us a lot of self-belief and the ability to express ourselves in a better way.
“We had a shaky period under Keane where there was talk of relegation. But we’ve started playing much better football.”