Originally, Kakashi had not thought much of it. He had only wanted to bring "Sasuke" back to Konoha. However, after meeting "Sasuke," he felt that this "Sasuke" was very unusual, so he did not rush to act. He merely watched "Sasuke" fight Shikamaru and the others from the side, which deepened his suspicions. As one battle after another passed without him making a move, he carefully observed everything. Finally, what had been mere speculation became certain: the person before him was definitely not Sasuke—or rather, not the Sasuke they knew.
The reason was that this "Sasuke's" fighting style was completely different from before. The old "Sasuke" had never used Shadow Clones in battle, much less the Rasengan. Moreover, this person's personality was completely unlike the Sasuke of old. They were practically two different people.
"Are you... really Sasuke?" Kakashi asked Chen uncertainly.
"Hmm? Why would you say that?" Chen was stunned for a moment after hearing Kakashi's words, then replied with amusement.
"When I first saw you, I felt that something was off, but I couldn't put my finger on it. I thought you had changed after following Orochimaru for so many days, so I didn't think much of it. But after what happened next, I couldn't help but grow suspicious."
"What are you suspicious of?"
"First, as far as I know, Sasuke has never used Shadow Clones in battle. Yet you used Shadow Clones several times while fighting Naruto and the others just now. Second, you actually used the Rasengan during the battle... And lastly, you gave me the feeling that you were extremely sly. You were nothing like the Sasuke of before."
"You can determine that I'm not Sasuke based on just that?"
"Of course not. What made me most suspicious was your attitude toward Ino." Kakashi spoke