Regardless of how Welsh plays for Celtic, there's absolutely nothing wrong with saying he was shite for Morton: that's an undeniable fact to anyone with the slightest understanding of football who watched him.
I reckon he might still turn out to be excellent centre back, but he's clearly not a right back and as that's where he played every week he was dreadful. Even in the unlikely event he goes on to play at the highest level and win the Champions League several times, it won't change that fact.