\'soft\' eastern soldiers

[Nathan Ross]

From what I've read, in Josephus's book, in this case it was the fault of the commander Cestus, who ordered a retreat even though he was poised to win, and this led to a rout.

Exactly - poor leadership rather than poor legionaries!

The decision to send western soldiers was made after the defeat of Aufidius(?) and his Syrian troops in 162.

You probably mean the defeat of Sedatius Severianus at Elegeia in AD161. Dio describes it as follows:

Vologaesus, it seems, had begun the war by hemming in on all sides the Roman legion under Severianus that was stationed at Elegeia, a place in Armenia, and then shooting down and destroying the whole force, leaders and all... Lucius, accordingly, went to Antioch and collected a large body of troops; then, keeping the best of the leaders under his personal command, he took up his own headquarters in the city, where he made all the dispositions and assembled the supplies for the war, while he entrusted the armies to Cassius. (Cassius Dio, 71.2)

Lucian tells us more about Severianus, who he calls a 'foolish celt' - apparently he was hoaxed by the charlatan prophet Alexander of Abonoteichus, who told Severianus that he would win an easy victory against the Parthians. Severianus therefore advanced without due care, and lost his legion... Once again, a poor commander was to blame.

Note that Dio has Lucius collecting his troops after arriving in Antioch - in other words, his army was largely composed of the legions already in the east. These troops (after, presumably, a bit of polishing!) go on to win great victories - again, the quality of the commander(s) is key. There were 'western' legions in Lucius' army as well, although only I Minervia is attested in Armenia, plus a collection of 2nd-century tombstones from Zeugma recording men from Danube legions, possibly victims of the plague. The Danube, with its great concentration of legions, was a ready depot of troop reinforcements, and the presence of these soldiers in the east should not necessarily reflect a lack of confidence in the men already there.