This is not a matter of who should be replaced to defend it. Although it is true that Jingzhou was lost in Guan Yu's hands, even if Zhuge Liang and Zhao Yun were replaced to defend it, it would not be lost in a short time.
Why was Jingzhou lost? Because when Guan Yu was fighting Cao Wei on the front line, Lu Meng of Dongwu attacked Jingzhou from behind. The main reason why Lu Meng and others were able to sneak attack Jingzhou was because the Shu Han was too weak. Even though it was already the heyday of the Shu Han's strength, its military strength was simply not enough to cope with Guan Yu's efforts to guard many counties in Jingzhou while fighting Cao Wei. Even when Jingzhou was lost and Guan Yu was defeated in Maicheng and died, Liu Bei in Hanzhong and the generals from all over the country were unable to come to the rescue.
It is true that it is far away from Jingzhou, but it does not deny that the Shu Han military strength at that time was indeed insufficient to guard its huge territory. If Guan Yu was fighting Cao Wei on the front line at that time, Jingzhou could be guarded by any general such as Zhao Yun, and the result would probably be different.