To be fair, I don't think Crassus was as bad a commander as hindsight and the Romans' need to find a scapegoat tend to suggest. He'd performed well in the Sullan Civil War and Spartacus' Rebellion, and the next big invasion of Parthia, under Mark Antony, wasn't exactly a stunning success. Pompey might be able to avoid being defeated as heavily, but I don't think sweeping conquests are on the cards, either. Most likely he'd advance into Mesopotamia, the Parthians would harass his forces whilst using their mobility to avoid getting drawn into a decisive encounter, and the Romans would capture a couple of cities, garrison them, and then declare victory and go back home with whatever loot they'd managed to acquire.