In that specific role yes, but on the whole, he left much to be desired. We all love Zanetti but he was not a great midfielder.
Zanetti was what Vecino should ideally be - an un-spectacular but solid and consistent midfielder who gets the absolute maximum out of his physical traits to partially make up for his lack of technical skills. At this point, Vecino can only dream of being reaching half of Zanetti's level. I do believe Zanetti was as solid a 'backup' midfielder as one could possibly be. The problem was, he was played far too often to be considered a backup.
It is one of the biggest mysteries and regrets in my time supporting this club that Zanetti was misused for so long in midfield when he would have been an ideal permanent LB solution. Zanetti's range of passing and forward passing was always pretty average, as was his ability to contribute to attacking, possession play in midfield. He excelled at dribbling and running with the ball as far as attacking goes and it was clearly visible he felt most comfortable in wide areas of the pitch, not central.
He lost his RB spot after Maicon came, that's fair considering Maicon was a devastating attacking fullback and it was simply impossible to not play him. But as good as Zanetti was at covering for Maicon, let's be honest - that's a side-duty that Stankovic or someone else could have done, not a priority and a role worth dedicating an entire player to. If we'd bought some better midfielders, we wouldn't have needed to "spend" Zanetti in midfield. Having a midfielder in Mancini's fucked system who was defensively strong but contributed to little in offense was a big part of the reason we were so tedious from 2004-09 and disappointing in Europe, but having 3? Jesus Christ. Think of how much more effective we could have been if we had Zanetti at LB instead of fucking Favalli, Grosso or Nagatomo and had a better-balanced, better-assembled midfield instead of playing Vieira, Dacourt, Zanetti, Cambiasso and Stankovic all at once.
Considering the ludicrous issues we've had at LB over the years, I really think Zanetti was overall, by far our most solid LB on average, barring some patches from Nagatomo, Maxwell and Chivu. Makes me sad and angry to remember we wasted an LB of Zanetti's quality doing supervision work in midfield while blowing millions and rotating through countless crap LB's, something we're still currently doing.