At the Siege of Naples, Belisarius had only 8000 men; he would have 10600 at Rome the next year, so not a big difference in numbers for him, but hey, its something.
Furthermore, unlike with Rome, which immediately accepted Justinian's rule, the citizens of Naples decided to resist Belisarius, and formed a militia to fight alongside 800 Gothic soldiers. Now, Naples did eventually fall, but in large part due to the failure of the Gothic king, Theodahad, to come to the assistance of the city in time, leading to his overthrow by Vitiges. Belisarius would later go on to defeat Vitiges in the Siege of Rome due to having the support of the Roman citizens and thus the ability to quarter his army behind the protective walls of the city of Rome; in an alternate world where Theodahad does come to Naples, Belisarius would not have this liberty. At Rome, Vitiges did have 25000-45000 men which Theodahad would likely also have at Naples, thus Belisarius would not only be heavily outnumbered but would be out in the open to Gothic attack as well.
Now, we all know he could just pull off another spectacular victory like the Battle of Dara or Ad Decimum, but this is alternate history guys, we're not just going to make some small change to the world only to get the same end result. For the sake of the POD, Belisarius loses, and is killed in battle. Whether Narses also lands in 538 is up for you to decide, but lets say he does; in my opinion, given his measly force of 7000 men, which was intended only as reinforcements, he would lose, but potentially this untested commander is given an actual force, do you think he wins or loses?