Metellus Numidicus who was replaced by Marius -using his influence among the publicani- was a competent commander. Also, Sallust´s accounts on the battles in Numidia show that they were bloody and Marius sometimes won with great difficulty. And Sulla did capture Jugurtha.

As for the German threat. In the years before Marius many incompetent consuls had destroyed their armies in Gaul. In 106 proconsul Caepio didn´t want to co-operate with a lowly novus homo. I think that probably 40 000 men died at Arausio -i can´t believe that the srmy there was 80 000. There was panic in Rome after this and Marius was elected commander -by the equites who thought he was the best. In reality the German threat was not that serious. If the Germans could muster 500 000 men the Romans could get 200 000 -at least.

Marius had only an army of six legions in Gaul where he defeated ca. 50 000 Germans. And he had some very competent subordinates; Aquilius, Sulla, Sertorius etc. A year later he and Catulus defeated the Germans at Vercellae. The Romans probably had at least 40 000 men against the German 75 000.

Marius certainly was a good general who had the skill of detecting abilities in his men. Also, his reorganizing was significant -though the process must have begun earlier.