Although Umaga seems to be doing well at Carnegie, when was the last time he really played at 10?
As I understand it, he's played full back all season.
While it's impossible to be sure, I wouldn't necessarily feel comfortable about a 20 year old who's been playing a lot at full back being our back up 10.
I think a lot depends on how close Jimmy is to fitness. If he's just a few weeks away, then I'd be ok using MlB or Miller as cover.
If he's 6-8 weeks away or more then maybe a rethink is needed