McLaughlin had been made Leinster's stand-in captain for the World Cup and the 31-year-old's final appearance in the GUINNESS PRO12 came as recently as September 4.
Cullen told the Irish Examiner: "It is a huge loss for Leinster. We picked him as captain and that is a reflection on what his peers and his coaches think of him.
"He is just a really good character to have around the place. He always played the game full on.
"It was tough news to try to process, but we have tried to get the right framework in place in Leinster, where players are well taken care of.
"He came off in that Edinburgh game [on September 4] with a head knock, he said during the game he did not feel quite right. He saw a specialist after that and was given the advice; the recommendation he ceased playing and Kevin has taken that advice on board."
To buy tickets for the GUINNESS PRO12 Grand Final click here. Also follow us on Facebook, join the conversation on Twitter, sign up to our YouTube channel for extensive match highlights and sign up for our newsletter for regular updates on the GUINNESS PRO12.