https://celticswire.usatoday.com/2019/05/15/report-ricky-rubio-has-interest-signing-with-the-celtics/He talked about his playing future in an interview with EuroHoops, and mentions a few possible destinations including Boston. He does acknowledge that a lot could depend on other moves that happen this summer with teams (like Kemba in Charlotte, or of course Rozier and Kyrie in Boston, etc.)
I'd love Rubio and I think he'd fit this team perfectly. He'd essentially be a replacement to Rozier coming off the bench and facilitating more for the other top guys. Unlike Rozier who said he "sacrificed his talents to give Kyrie + Hayward shots", Rubio would actually play to his role and his passing would help us a ton.
I think Rubio could fit in any sort of plan. If we lose Kyrie in FA and don't trade for AD, we could sign him to go along with Smart, Brown and someone we draft at the guard position. Or if we keep Kyrie and do trade for AD, we could still sign him since it's likely we lose one or both of Smart/Brown in the trade so we could still use some added guard help.
I'd prefer using the MLE on a rotational big, but if we end up pulling off an AD deal then I wouldn't mind using the MLE on a backup/depth guard like Rubio.