Sasuke earned the elegant nickname "Sunny Day's Assistant" because of his battle with Killer B, the Eight-Tails Jinchuriki.
At the time, he had awakened the Mangekyō Sharingan and swaggered into Kumogakure with three followers to challenge Killer B at his doorstep, only to get thoroughly beaten.
Genjutsu was useless, his Lightning Release was inferior, and after all his flashy Taijutsu finally landed a direct kick, the other man's clothes were barely dirtied. His attention was even focused on the rap lyrics he had just thought of, and he hurriedly pulled out a notebook to write them down—
If Karin had not been there to heal him, Sasuke would have been the first to get skewered into a twisted mess by Killer B's swordsmanship.
Without Suigetsu and Jugo's support, the Sasuke of that time would not have had enough lives for Killer B to kill—
In the end, the entire Eagle Team fought desperately and thought they had killed Killer B, only to discover that he had used a severed octopus tail from the Eight-Tails as a decoy.
That little trick had fooled the Sharingan, famed for its insight! The Mangekyō Sharingan!
And the most dangerous exchange in that battle had undoubtedly been Killer B's ferocious Lariat, which directly blasted apart half of Sasuke's torso, exposing his ribcage.
Wu Yu had a very deep impression of this ability of Jugo's.
His Sage Transformation could actually activate flesh to such an extent. It was as though his body had become clay, able to change shape at will and be injected into others to repair their bodies.
If he remembered correctly, Jugo could even forcibly take other people's flesh to replenish his own lack of "body resources"—after Jugo gave Sasuke some of his flesh to fill the hole Killer B had punched through