Must and Can in prediction_

Must and Can in prediction ?

1) " I am sure it isn't the engine -it's just been checked."
Rewrite: It *CAN'T* be the engine - it's just been checked"

2 ) "The patient is not recovering well after the operation. He *MUST* be frustrated "

At 1st, do you think the use of must and can in these sentences is ok ?
And, if they are correct, it seems that both "CAN" and "MUST" are to use for expressing a prediction. So what is the difference between them ?

Thanks and sorry for bad English .