As fans must know, the key reason behind Seishiro Nagi's downfall in Blue Lock was that he was no longer fired up. He had no real goal he wanted to pursue and lacked the inspiration to do what he did best, miracle plays. In fact, he needed Reo's help to even find a new fire to help him surpass his rivals.
Due to this, many fans believed that the person responsible for Nagi's downfall was Yoichi Isagi. However, the manga's latest chapter finally saw Ego Jinpachi confirm who was responsible. Shockingly, the person responsible for Nagi's downfall was the one who wished to see him blossom, Reo Mikage.
Disclaimer: This article contains spoilers from the Blue Lock manga.
Ego Jinpachi blames Reo Mikage for Seishiro Nagi's downfall in Blue Lock

With Seishiro Nagi being ranked 24th in the Neo Egoist League, fans knew it wasn't long before the striker would be "locked off" from Blue Lock. As expected, in the manga's latest chapter, Ego Jinpachi asked the players who weren't ranked in the top 23 to "lock off," meaning their time at Blue Lock was finally over.
Just as the Losers Gate opened, Yoichi Isagi asked Ego Jinpachi why Nagi did not qualify. As per Ego, while Nagi had some good moments, the disparity between his good and bad plays was too large. Hence, his salary reflected his abilities.

That's when Reo Mikage intervened. He asked Ego Jinpachi to redo the calculations as it didn't seem right for a "genius" player like Seishiro Nagi who was acknowledged worldwide to disappear at such a stage.
That's when Ego Jinpachi retorted explaining why Seishiro Nagi deserved to be "locked off." As per him, Nagi's talent had withered away. Talent wasn't the power to believe in your abilities but to prove them. During such pursuits, people often surpass their talent and come under the impression that the fluke moment was their real power.

This is what happened with Seishiro Nagi as people created the illusion that his five-feint volley against Bastard Munchen was the extent of his abilities.
Amidst this, Nagi's biggest mistake was scoring that goal. Having scored the miracle goal by a fluke, Nagi believed he accomplished his dream of winning against Isagi. That moment effectively satisfied Nagi. Hence, he no longer possessed the fire required to hone his talent.

Realistically, Nagi could have kept his fire burning. Even if had lost, he should have tried to prevail by himself. Amidst this, what truly killed Nagi's talent was the decision to team up again with Reo Mikage. Even though Nagi's talent had withered away, Reo kept watering it, hoping it would blossom again.
While Nagi talked about becoming the world's best with Reo, he couldn't find a fire beyond the dream of defeating Yoichi Isagi. Hence, as harsh as it may sound, teaming up with Reo caused Nagi to get "locked off" from Blue Lock.
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