From the outside, apart from the minor injuries inflicted by William and the others which amounted to nothing for a monster this big, the Satcthalle seemed to be fit and in good condition. However, when Lucas decided to change his 'vision' and sense the monster's breathing and energy movements, he noticed that there was a clog.
This meant that the Satcthalle's injury wasn't an external one but an internal injury.
From his experience, Lucas was also able to surmise that the Satcthalle got the injury from two possible experiences. The first was while trying to forcefully break through to the next level. As for the second, it could have been from a severe injury sustained during a difficult battle. While the mark might have healed, there was a hidden trauma that birthed a latent injury affecting the Satcthalle.
'This is better.' Lucas thought as he slowly exited from the darkness.
