Barcelona will be determined to achieve victory over Porto, because Xavi wants to ensure his club progresses without further delay. At the same time, Shakhtar Donetsk will try to secure full points from Antwerp in front of their own supporters. Automatically, if the scenario goes like that, then it is true as you said, the determination to qualify for the round of 16 between the two clubs will occur on matchday 6.
However, it could be that this opportunity only works on paper, and Barcelona could even stumble or be forced to share points by Porto, as well as Shakhtar. So the determination to qualify for the round of 16 will continue in the final match of the group phase, I think this is also very open to happening, because we have seen that Barcelona can also lose surprisingly.
In my opinion, Xavi squad should be able to qualify easily without encountering any problems, because we know their history so far in the Champions League. But lately it seems like they are less of a favourite, that doesn't mean I underestimate them, but I see that Madrid and Bayern and Man City are highly favored by most football observers.
The defeat achieved in the 4th Match made Barcelona now under pressure, they only had to achieve a draw in the match to qualify, Now Shakhtar Donetsk chances of qualifying are alive again after beating Barcelona. Barcelona chances of qualifying are still very big in the remaining 2 matches, especially H2H and their goal aggregate is very good. Barcelona only needs a draw against Porto to ensure they qualify for the round of 16, even though Shaktar won over Antwerp to increase their points to 9, they have to face Porto and Barcelona will still qualify whatever the result. But if Barcelona loses to Porto, they must win the final match versus Antwerp to qualify.
Barcelona can still win the group if they get at least 4 points from the last two matches. Xavi must be able to achieve the target of qualifying as group winner to get an easier opponent in the round of 16. Against Porto, Xavi needed to put aside the experiments that were often used in the last few matches, because Xavi often tampered with the Barcelona squad, causing them to lose a lot of points in La Liga, including the draw they achieved versus Rayo.
Xavi must seriously prepare his team against Porto, he must do his best to avoid defeat so that his team is not under pressure in the last match against Royal Antwerp.