We are always open to attack somewhere, especially as we deliver a punch. We can however minimise the openings by use of opposite cover, or protecting ourselves as much as possible with the hand/arm that is not delivering the strike.
This basic principle is one of the most important parts of defence.
Basically, as a punch is delivered, whichever hand is “on them”, the other one has got to be “on you”.
You will see some fighters, gifted with elite reactions, not always applying this, or even boxing with low hands in general. However, they often have shorter careers that end with receiving a lot of knockout losses as their reactions decline.
We all get tired in training & find it hard to maintain our form, but drilling this consistently could make all the difference.