That part you pulled off is call the wave guide cover. This has to be in place for it to work probberly.
It will still work without it but food debris and water and splash through the gap and into the magnetron area so this will need to be replaced.
Now sometimes if its ripped in half then it can actually start to spark on the cover so I wouldn't use it with just half of it to be honest.
You may be best contacting whirlpool as its new and see if they can send one out or if you want the part number for a new cover its 481246228268
If you need anymore info just get back to me.