Lascelles revealed dissent in the ranks under McClaren, who was sacked in March 2016 with the Tyneside club mired in the relegation zone.
"The players' heads were all over the place back then," Lascelles said. "It (dissent) was there. It was bad. Steve is obviously a well-respected manager but there is only so much you can do when you have 20 players refusing to do things and not taking anything seriously."
Lascelles has become a fan favourite at St James' Park for his commitment to the cause and refusal to tolerate anything less than 100% from his colleagues. In October, he had a training ground bust-up with team-mate Mo Diame after confronting the Senegalese about a perceived lack of effort.
"It's my job to tell people," Lascelles reasoned. "If I didn't I wouldn't be wearing the armband. If I wasn't pulling my weight I would expect to be told. So I am not worried about upsetting anybody."
It was this competitive edge which prompted current manager Rafael Benitez to appoint Lascelles as club captain after Newcastle's relegation to the Championship in 2016. Lascelles, who describes Benitez as "world class", believes that the club's dynamic under the Spaniard is completely different to the previous regime.
"Because of the respect he [Benitez] has, players wouldn't dream of going against what he says," Lascelles explained. "I wouldn't let them do it either. This is totally different now. No cliques. All together. I don't think for one minute we will go down this time.
"I want to win. We actually have a great group of lads here now. They give everything."