That goes without saying

Warlock sighed at the memory, secretly scolding himself for caring too much about the little creature. But then he consoled himself again, telling himself that he was saving humanity by preventing Presley from gaining strength too quickly.

And knowing that the little creature had heeded his advice and was now safely with Juin, he sighed in relief. At the very least, until Juin was strong enough to deal with the dangers by himself, that little creature would be there to protect him. 

He had never mentioned this to Lucifer and Juin, but it seemed that when he crossed over, he was either too strong for this world or he was being punished for bringing the soul of Hades over. Not only did he suffer a backlash, and that his powers were halved, but it was also harder for him to gain strength.

Thus, he would only be able to protect Juin up until a certain point. The boy had to grow stronger quicker, and smarter, if he wanted to escape his destiny.