Weak as she was, Navi wasn't sure she was in any fit state for the sort of power the cat was offering her. She'd gladly accept it, only a fool wouldn't, but she was in pain, and above that she was exhausted.
It must have been obvious, because even as Grendal waited for her answer he turned to the new senshi that he'd help to awaken, an ear flicking back in consideration. Ares had requested they go to the hospital, but maybe there was a better way. Virtus might now have been a senshi of chaos, but his powers reflected his sphere of kindness. His 'attack' was healing, and after repeating the spell from Grendal and laying hands on Navi the worst of her injuries had faded.
With the immediate concern for her safety gone, Navi was free to accept the offer of power from the guardian cat, and with him, along with her Queen, Navi ascended to her super form.

(DONE! Not the best wrap up, but it covers all the bases.)