The cliffhanger that Jujutsu Kaisen Modulo Chapter 15 was an absolute gut punch. Yuka Okkotsu is suffering from terminal brain cancer, so she obviously can't do anything but use the Ten Shadows Technique. She jumped out of a plane, used Mahoraga against the alien warrior named Dabura, and immediately after summoning, she seemed to just disappear into the shadows. It was totally a smart move because it saved her, also doomed her too.
Yuka’s withdrawal into the dark is not merely a tactical retreat; it’s also a way to recall Megumi Fushiguro’s infamous Shibuya moment from Jujutsu Kaisen Modulo when he summoned Mahoraga to save himself from Haruto Shigemo. Only because Yuji Itadori intervened at that point was Megumi spared.
In Yuka’s case, however, she is stuck between an uncontrolled divine general and her illness. It provides a context in which this original protagonist can return meaningfully again in Jujutsu Kaisen Modulo.
The Ten Shadows Ritual Explained

The way that Yuka conjured the summoning ritual was in accordance with the established way of brutal summoning in the original Jujutsu Kaisen. A Ten Shadows user must beat or be beaten by an unexercised shikigami to be able to exercise it.
This also applies to Mahoraga, the ultimate shikigami; he has never been exercised from the beginning of time. If either one is eliminated during this time, too bad for one of them; no escape clause here.
According to discussions, Yuka cannot simply dismiss Mahoraga once summoned. As it's been pointed out, Megumi specifically stated "I'll be dying first" during his attempt, implying the shikigami doesn't unsummon upon the user's death. Therefore, it is even more dangerous for Yuka since he has terminal brain cancer.
Hiding in the Shadows: A Temporary Reprieve in Jujutsu Kaisen Modulo

Yuka was smart to go into hiding after she summoned Mahoraga because this is something Megumi didn’t do during the Shibuya raid. She used being out of the field to give herself time and create an advantage. But the only thing this does is delay the inevitable – eventually she has to come out of hiding and, when she does, she'll have an uncontrollable Mahoraga waiting for her.
Jujutsu Kaisen Modulo Chapter 15 highlights this predicament perfectly. It's noticed that Yuka is in a state of suspended animation, meaning she can't get herself out. In this case, there will need to be an outside force that intervenes. So, unless someone defeats the Mahoraga or helps Yuki escape from the dark place, she is going to die from one of two "certain death" options: the Shikigami or her advancing cancer.
Why Yuji's Intervention Makes Narrative Sense

Gege Akutami has demonstrated throughout Jujutsu Kaisen Modulo that he understands how to incorporate legacy characters without overshadowing the new cast. Yuji's cameo in Chapter 6 was done with a sense of proportion; he acted as a reference point instead of taking over the attention of the audience. Instead of just bringing Yuji back to make corny old fan service, it'll be practical and serve a purpose in the storyline.
So basically, Yuka has put herself in a no-win situation by going up against a fight against a monster even Sukuna had a hard time beating. They are still going at it, and Dabura has at least one technique that human beings do not have a chance of understanding at this point.
Things just got a little crazier as Mahoraga is now adapting to a new curse technique from an alien for the first time in history. No one will be able to interject in this ritual without setting off full-on war with humanity and the Simurians unless they have the power and experience of Yuji.
Instead of taking out Mahoraga, Yuji's auxiliary part in Jujutsu Kaisen Modulo may be only to revive Yuka out of the shadows or provide a means to allow the ceremony to end without her death while saving her. Rika's existence, tied to the Okkotsu family (via Tsurugi's ring), is a factor in attempting to rescue Yuka.
In Jujutsu Kaisen Modulo, the desperate situation in which Yuka finds herself is ideal for bringing Yuji Itadori back into the mix with purpose. Because of Yuka being trapped under Mahoraga, who is violently attacking the surface world and dying from terminal cancer, she needs exactly what Megumi needed when they were at the fight at Shibuya.
Whether that miracle comes in the form of an elderly Yuji, the emergence of Rika, or some other unexpected development, the stage is set for one of the sequel's most significant moments yet.