In the Infinite Tsukuyomi, Rock Lee was able to beat Naruto and Neji. All without using ninjutsu and genjutsu. Just taijutsu. Tenten was cheering him on and patting him on the back. His sensei, Might Guy was cheering on and declaring him the true embodiment of the youthful spring. But the best part of it all was Sakura, wrapping her arms around him and declaring her love for him. Oh, yeah. Life was good!
(!)
In the real world, Lee, along with everyone else, was cocooned and tied to the Divine Tree's roots. Kaguya smirked at this fool's dream. No ninjutsu or genjutsu, he wasn't worthy of being a ninja. Yet, as a White Zetsu, he would do better.
(!)
The first thing that disappeared was his clothes, exposing his muscular frame. The result of years of rigorous training. He won't need them.
(!)
Very slowly, his chakra was drained, robbing him of all the strength and taijutsu his body had accumulated over the years.
(!)
Years passed. Lee's chakra was almost gone. By this point, his muscles had receded and his skeleton was starting to poke through the skin.
(!)
As his body was being drained, his mind was going, too. The teachings of Might Guy were being forgotten and so too was his philosophy of the Springtime of Youth. In place of Might Guy's teachings were the commands of a female. She was the epitome of the Springtime of Youth because she would never age and never die. She told Lee it was his job as a soldier to preserve that youth for all eternity. Lee listened to it all and not once did he resist.
(!)
More years passed. What was left of Lee was now nothing but a withered husk. But just like everyone else caught in the Infinite Tsukuyomi, he was kept alive and preserved. Now his body can begin the second phase.
(!)
First, his muscles started to grow back until he had the build of a proper soldier.
(!)
Months passed and by this point, his skin turned white with jagged lines on it. His black bowl cut turned into spiky green hair and his eyes turned from black to a sickly yellow.
(!)
A few more months passed. Finally, the cocoon broke and the White Zetsu inside fell to the ground and joined all the other White Zetsu. All of them equal in appearance and in strength. Lee no longer had to worry about being a ninja with no ninjutsu or genjutsu; just taijutsu. Now, he was White Zetsu, a soldier with one purpose: to protect Princess Kaguya and her world. For she was the eternal Springtime of Youth. It was his job to preserve that youth no matter the cost.