And fellow live wire Chad Wingard looks likely to miss Saturday's match between the Power and Lions with an ankle injury.
The Power's 'no risk' approach to injuries will come into play with Wingard who, like Monfries, exited Sunday's seven-point loss to North Melbourne at Etihad Stadium during the third quarter.
Both injured themselves in separate incidents while chasing the ball in attack.
Port Adelaide's midfield assistant Josh Carr confirmed the outs at the Power's closed training session at the Adelaide Oval on Wednesday morning.
"[Monfries] is out for about 3-4 weeks and Chaddy Wingard looks like he probably won't come up for this weekend's game," Carr said.
"We'll go into the game, definitely, with a no risk policy.
"If he [Wingard] can't get up, he won't play."
Carr suggested Aaron Young and Paul Stewart could be considered for a call up after playing good games for the club's Magpies League side last week.
Port Adelaide will announce its team for Saturday's game against the Lions on portadelaidefc.com.au at 5:55pm on Thursday night.
