I personally feel this is illogical, inconsistent & terrible writing. Even if one were to lowball Goku completely and consider Roshi to be the strongest human, there is absolutely no justification as to how one can scale Roshi's power to even Base Goku, let alone Super Saiyan Goku. Scaling him to Super Saiyan Blue Goku in terms of power again is just absurd.
However, with an obvious degree of difficulty, it is possible to justify the same for the following reasons.
- Firstly, Roshi is definitely not using Ultra Instinct. If Roshi has any notion of Ultra Instinct which even Beerus can barely use, it would make no sense. I believe, Roshi has watched and studied Jiren's attacks during the course of the tournament and is simply countering his moves similar to what Vegeta did in Episode 122.
- In terms of power, Jiren is nerfed in the manga. In Manga Chapter 37, where Mastered Super Saiyan Blue Vegeta fights Beerus. After Beerus one shot MSSJB Vegeta, he said that Vegeta was strong enough to be a God of Destruction in another Universe. Also in the God of Destruction tournament, Beerus's suppressed attacks were supposedly strong enough to kill Multiple Gods of Destruction. Hence, that would already bring Goku and Vegeta to God of Destruction Tier and hence, the power difference between Jiren and Goku/Vegeta, might not be as big as it was in the manga. Note: I know this still doesn't justify the fight with Roshi, however, I thought it might be an interesting point to include.
- Jiren is not as skilled of a fighter as he seemed to be in anime. He only possesses brute strength and raw power. This might be hard to believe based on what we see in the anime, however, it would be a logical explanation as to why someone like Roshi is capable of dodging Jiren's attacks. We see a worn out Goku being able to do the same while fighting Kale and Caulifla. Hence, Roshi might be a superior Martial Artist and has the edge on Jiren.
- In Manga Chapter 29, where Toppo mentions Jiren, he stated that Jiren surpassed Vermouth in terms of Battle Power Alone. Also, in Manga Chapter 36, when Toppo fights Vegeta, he stated that in terms of sheer power, Vegeta was on par with Goku. Although I'm making an assumption here, I think one could logically use this to argue that Toppo perhaps meant while Jiren had the raw power superior to Vermouth, he possibly isn't superior in terms of skill and the same applied to Vegeta with regard to Goku. This could perhaps be used to defend Roshi's fight with Jiren
- In the anime, when Vegeta keeps persisting and pushing back Jiren in Episode 122, Vegeta stated that Jiren was using less power and speed than what he used against Goku. Also in the early phase of their fight, Jiren did seem very uninterested in fighting Vegeta and kept considering him to be persistent and annoying. This technically lead to Jiren lowering his guard enough for Vegeta to land 2 solid blows. Jiren might possibly feel the same way about Roshi and substantially lets his guard down. If you watch Episode 122, Jiren initially treats Vegeta like a fly and simply just swats him away twice. He might be possibly trying to do the same against Roshi who obviously is putting a lot more effort and trying to capitalize on the same.
- Even if you look at the scans, Jiren doesn't really seem hurt or injured or anything as such. He seems more annoyed than anything and is obviously uninterested in fighting Roshi and simply trying to knock him away(While completely underestimating his fighting experience).
- Finally, I highly doubt this is a reason, however, Jiren might possibly be afraid of killing Roshi and possibly finding it hard to suppress himself to a level capable of knocking out Roshi without killing him.
In Conclusion, characters in the Manga are power scaled significantly different from the anime. I think it would be best to not compare the Manga characters with the Anime characters anymore. I believe the 2 best possible reasons to Justify the same, first in favor of Roshi, is that Jiren is nerfed in the manga; probably not in terms of power but in terms of skill at least and Roshi might hence be a better technician. In favor of Jiren, he simply considers Roshi annoying and has no interest to fight him and simply wants to push him away without hurting him(Hence, severely underestimating his skill) and Roshi is capitalizing on the same.