DBRS, the last credit rating agency that still has Portugal’s government bonds rated as investment-grade, will have a chance to adjust its rating. The European Central Bank’s (ECB) rules dictate that it can only purchase bonds with an investment-grade rating. Should the DBRS change its rating, Portugal could be closed out of the ECB’s bond purchase programme.
Of course, if the ECB would stop buying Portuguese government bonds, it would increase the bond yields and lead to a new crisis.
Fortunately, Portugal could always apply for a bailout (no new money, but being under a Troika programme and regular review, Portugal would then be eligible for the ECB's purchases).
If this would happen, the initial reaction would be panic-selling of Portuguese banks&bonds, only for them to bounce back later when a deal is formed.