I had this happen to two of my lambs this year. The first I splinted both front legs and gave it physio each day several times a day. After ten days there was not change and sadly the vet had to put him down.
The second was not so bad and I got it onto grass as soon as I could. I splinted both legs for two days and then let it fend for it self without splints whilst stretching the muscles out when i could. I thought that it was not going to improve but it bounced back and this evening, three weeks later, it is running around with the rest of the gang with no issues at all.
In short if it does not improve in the first few days I think it will be fairly terminal.
Good luck.