Uchiha Tajima grew serious.
In all fairness, Sasuke Uchiha's fundamentals were far from poor. In fact, they were outstanding among his peers.
He had received the Uchiha Clan's elite training since childhood, and after the Uchiha Clan Massacre, he had endured brutal trials while training alongside Naruto. His foundations in taijutsu, swordsmanship, ninjutsu, shurikenjutsu, and more were all solid.
But one point of comparison made Sasuke's solid foundations seem somewhat inadequate: Naruto Uzumaki.
As a child, Naruto had undergone systematic, relentless training in the fundamentals of taijutsu under Might Guy's almost punishing standards.
But Naruto felt that focusing on taijutsu alone was not enough, so he worked on both fronts.
His days were filled with hellish physical conditioning and taijutsu training, honing every movement until it became almost instinctive. At night, he practiced chakra control with mind-numbing repetition, progressing from water walking and tree climbing to ever finer control.
He could even use chakra threads in place of steel wire to control shuriken in flight.
Those years had been crucial to building Naruto's solid foundation.
By comparison, Sasuke worked hard as well, but his growth had been driven more by bursts of genius and an obsession with revenge. When it came to the painstaking work of systematically and continuously refining the most basic skills, he had indeed devoted less time and focus than Naruto.
Fundamentals meant more than mastering a few ninjutsu or knowing a few sword moves.
They encompassed basic physical fitness, instinctive combat reactions, the most efficient paths for chakra circulation, the simplest and most effective ways of generating force, the steadiest control over one's mental state...
All of these required immense amounts of time, accumulated bit by bit through countless repetitions, corrections, and real battles until they settled into instinct.
To a true master like Uchiha Tajima,