So short short answer (after all that!): To be a good/great improviser you probably need to be able to hear the music in your head, but don't necessarily need to be a good singer. Being able to sing obviously means that you can hear the music, but doesn't mean that you've got the physical coordination necessary to be a great player. Each can learn within limitations of talent/ability to sing or play.
Knowing your limitations and talents (self knowledge) is the key to being able to advance quickly in any pursuit. Determination can go a long way towards making up for a lack of natural ability, but can also be a depressing dead end unless you are enjoying the process.
You've struck a chord in me with those words Wade!
After many years of trying to improvise, avoiding scales, theory, buying many books & play alongs...even lessons..
It's all been very slow progress, and host of possible reasons why this elusive improvisation is always just around the next corner.
With the latest foray into Gary Burtons 5 week Introduction into improv...I am still struggling with theory.
Playing the changes...I thought would mean...that now I can really play harmoniously, freely and with instant creativity.
The more I go down this path.....without ear training...without being able to hold a phrase or tune in my mind and able to sing it.....I am now sure ...for me is the wrong way to go about learning to improvise......I've started to transcribe tunes by a mix of ear and writing down the notes (not on staff paper) and really copy the feel, the rhythm the swagger of it, by ear.
I am sure when I can't play it.......I find I can't even sing it....so I break it down until I can sing it...then eventually I can get to the point of finding the notes. Even looping a section and getting it near perfect, playing it slow and fast, thinking now I've got this, then if I have a 1 minute break....I find I can't remember the starting note...this is the point where I think, if I can sing it...I will have it.
Is this common amongst others or is my brain simply not very suited to improv........?
As Wade says above....... "can also be a depressing dead end unless you are enjoying the process"
I hope or assume that singing, may just be the magical key to unlock this improv door and open a new world of enjoyment!