Rubio is without a doubt a starting caliber PG. But he has a unique skill-set that, especially as the league has become more guard driven/perimeter oriented, needs to be specifically schemed around in order to maximize his performance/effectiveness.
Rubio, understandably, is struggling to adjust his game to the offense (which he is still learning), while being a PG who needs (more so than most) to be in a system that is adjusted to his. He needs shooters/space to operate, which our roster is currently unable to sufficiently provide.
He was a value pick-up at a time in which the roster was in flux. It may not be consistently apparent, largely in part due to DL essentially being forced to quickly patch together a competitive roster with pieces that don't necessarily fully fit/compliment eachother, but Rubio is still a young, talented PG who excels at passing & defense.
Although if we do commit to him as our long-term PG, the roster will need significant readjusting. But, to be honest, I don't see him as an ideal PG pairing for Gobert.
Either way, he is a much better PG than some people appear to be giving him credit for. I fully expect him to improve as he becomes more familiar/comfortable with the system/team.