But the 31-year-old defender says all that has now changed under new manager Brendan Rodgers.
"It can be a little difficult to take as a player mentally," Monk told the BBC's Sport Wales programme.
"The methods in training, there was no real reasoning behind any of his methods and why we were doing things. There was no great organisation towards it.
"Knowledge-wise he knew his stuff. But it was hard to take when you've gone from one extreme [under Sousa's predecessor Roberto Martinez] to the other extreme but it like we're back on track now.
"Training has stepped up and especially the way we play football, our football fitness is massive to us and I think you could see a lot of times last season we would dominate teams, get to the second half and we would be a bit tired and that mainly comes down from training."
Monk praised the work of Sousa in making Swansea's defence one of the tightest in the Championship last season, keeping 24 clean sheets.
But he admitted the two-times Champions League winner's style was not best suited to the way the Swans play.
Fantastic to hear the players reacting to Rodgers so positively and I'm so glad the camp is a happy one again
uplandstavernswan
"It was frustrating in terms of training going from working intensely hard every single day to practically not even getting a sweat on," added Monk, who had an on-the-pitch bust-up with Sousa after their draw with Crystal Palace last season for shaking hands with the referee who Sousa claimed had not shown the Swans respect.
"Every manager is different, everyone's philosophy is different. Paulo was a lot more laid back than other managers and you take each one as they come.
"As a group of players that I know and have trained with, every single day I know that's not what was best for us.
"Everyone agreed that we need to be pushed and pushed. In training we need to be going over the edge sometimes to get your fitness where you need it in the last five minutes, where we are being pushed to the limit in the game.
"That's kind of where we let ourselves down but it was difficult as well because we weren't allowed to do extra training.
"Every manager is different and you learn from every manager and defensively we learnt a lot from Paulo.
"We thank him for that, wish him all the best at Leicester but I think it was right for him to move on and now we've got a manager in who's going to work us."
Catch Sport Wales' preview to the new season on Friday, BBC TWO Wales at 1900 BST.
Source: BBC Sport
Source: BBC Sport