If Kyrie is back, the team will most likely have only the taxpayer MLE to sign a new player. Depending on if a Davis trade happens, Smart could or could not be back. There are a lot of variables.
Regardless of how it happens, if the Celtics have only the taxpayer MLE to sign Rubio, I don't think he signs for $5.7 million a year. He is too good for that. If the Celtics have only the standard MLE, I guess, regardless of role, Rubio might agree to getting that $9.2 million a year. I could see that happening as long as Rubio is given a strong role as a starting PG or 3rd guard off the bench.
Danny could make worse signings. Rubio would be a good add but not really a difference making one. But he would know his role, which is important.