As Smurph said, he's not playing because Vipotnik is playing and scoring at a level far exceeding his first season here.
There's no doubt in my mind that Idah was signed to be our main striker this season. A club like us doesn't spend as much as we did (wherever in the suggested fee paid it actually fell, it's certainly a few million at least) for a backup. If if wasn't for Vip's huge improvement, Idah would be the regular starter up top, despite the judgement of a few football experts on here that he can't play there.
Once Idah is fit, if Vipotnik is injured or sold, Idah will 100% take his place in the team. If he does so for a good run of games, we can then assess whether he's actually good enough or not, rather than make a ridiculous early judgement that is then clung on to despite how irrational that is.