Michael was under the impression Pentangeli [and Cicci] was dead

I can't see Michael abandoning the Corleone operation in New York, his muscle

It seems to me Michael appointed / elevated, the “talented” Joey Zaza, the long standing member going back to Clemenza's old regime and Michael was running the Corleone business from Tahoe by proxy until Zaza was officially awarded the business

Otherwise the Rosatos would not have been "on the run"
If Michael had not stepped in, it would have been a walkover for the Rosatos, to take over the Corleone operation, thrown into disarray with no Pentangeli no Cicci