There is no need for 'A surgery' .. Surgery alone is correct
Lanawee is correct
In fact, if you really want to use an indefinite article, you could say: "An operation"... This would seem to be better for the translation, too. Actually, more common in American English than "surgery."
Penting means important. A surgery is important for a severe illness.
Necessary means perlu. Necessary is not the same as important.
"Operasi perlu untuk peyakit parah"
In that case it should be "diperlukan"
Shouldn't "adalah" be included in this sentence because without it sounds like it says "Important surgery"?