Monfries appeared to get through the final match of the day in good health, but coach Ken Hinkley conceded the 26-year-old would miss at least next week's clash against Melbourne in Renmark.
The former Bomber impressed in against the Crows, collecting an equal-high 11 disposals and a goal, having kicked another goal against St Kilda earlier in the day.
Hinkley played down the injury, but conceded it would sideline Monfries.
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"We think he's got a little grade one hamstring - I'm not into keeping secrets, so I think that's what it is," Hinkley said.
"[It'll] put him out for a couple of weeks, but it's that time of the year when he's done absolutely every session of pre-season - we weren't going to play him in the next game anyhow."
Hinkley was pleased with the performances of most of the club's young players, with several leading from the front in both of the Power's games.
Jasper Pittard had eight touches and two inside 50s against the Saints, while Oliver Wines and Chad Wingard also looked at home with eight disposals.
Reigning Sandover medalist Kane Mitchell was lively and picked up 11 disposals against Adelaide, seven of which were contested.
"I don't normally sit here and talk about which ones were good and which ones were bad, but I think I might say that most of our young players today that went out there and played showed glimpses, which was pleasing for us," Hinkley said.
"With 11 new ones on the list it was just great to see them play for the first time."
Pocket-rocket Jake Neade's performance in game one suggested he has a bright future at the elite level, exciting the crowd with his acceleration, evasion and skills.
The 170cm 18-year-old from the Northern Territory looked at home at the highest level, finishing with three inside 50s and a goal before sitting out the mini-Showdown.
"Size normally doesn't stop you. If you've got effort and energy you'll certainly push yourself up and be a part of it," Hinkley said.
"Jake showed today that he's got some real positives about playing AFL footy."
Harry Thring is a reporter for AFL Media. Follow him on Twitter: @AFL_Harry.
