Porplyzia, who suffered brain bruising and concussion against Carlton in round 18, saw a neurologist on Wednesday with the hope of being given clearance to play on Sunday.
But despite his scans showing up clear Porplyzia was advised to stick to the initial plan of missing three weeks.
The talented 23-year-old, who is also nursing a shoulder injury, will rejoin competitive training next week and teammate Ben Rutten said the Crows would simply have to make do without Porplyzia against the Saints.
'It would've been a bonus if Jason was available because of the quality player he is, but we haven't had him the last couple of weeks,' Rutten said.
'With the type of football we're playing at the moment, it would be good to have a player like that, but we're managing without him.'
Young onballer Bernie Vince will also miss another week with a hamstring strain, but Chris Knights looks set to resume after completing Thursday's training session.
Last week's five-goal hero Nick Gill, who spent Saturday night at West Lakes receiving treatment on his troublesome calf, left the track early and will need to get through training on Saturday morning to prove his fitness.
The Crows left it until round 22 to secure their place in the eight last season, but this year, with 12 wins already to their name, are guaranteed a September berth.
Rutten, who will play his 100th game this week, agreed the club was better placed than last year, but said Adelaide wouldn't become preoccupied with just where it would finish the season.
"We're just keen to keep our momentum going and to keep playing the brand of football we want to take into the finals," he said.
"Where we finish in the end will depend not only our results, but on opposition results, so we're more interested in the style of footy we're playing."
Rutten has been linked to Collingwood in the Melbourne media, but the Adelaide vice-captain said he was bewildered by the rumours.
"I don't know where it's [the rumour] come from," Rutten said.
"I'm under contract for the next couple of years, so I don't know what else to say."
