Another cover letter question

BLUF: My site lead is grooming me to take over for my supervisor. My sup leaves tomorrow and the position was just posted last night.

Should I use a cover letter in my application? They are allowed by the site but not required when submitting. I know the hiring manager, he has asked me to apply and wants me to cover the position until they hire in the hopes that I can prove myself enough to justify putting my resume on the top of his list.

Mac