However, it is expected that the ECB will step in buying the Portuguese bonds.
I suspected that is why Draghi did not use the QE bazooka previously until last year having factored contagion effects from Greece. As you say they can keep buying Portugese bonds and keep yields low so continuing the theme of "cheap money" to quarantine any peripheral bond yield spike as a ratio of bund.
Technically, the referendum is a hindsight exercise as the default kicks in on Wednesday morning and forced upon ECB's hand regarding the ELA so Bank of Greece have no choice but to set withdrawal limit as the cash pie is now limited! What will ECB do next, political or economical considerations?