Truth to be told, Zhang Yang could have just taken another bottle of [Level 5 Healing Potion (Grade 10 Transmutation)] after that. He could still recover another 60,000 HP if he does that.
However, he would have recovered too much amount HP that he would have pushed Savage Lord into despair. When that happens, Savage Lord would have just given up hope and he might not use his third [Full Recovery Potion] if he really had one with him. Therefore, Zhang Yang had to maintain his HP bar at a certain amount to encourage Savage Lord that the match was not over yet. He could coax the other Guardian into giving his best and pulling out every other trump card.
Haih... more effort is required in order for one to be the villain these days.