The San Francisco 49ers have arguably the biggest game of the season on Monday, as they take on the Baltimore Ravens. However, the defense could be in trouble, as it sounds like Arik Armstead is still struggling with injury issues.

As a result, Armstead is already ruled out for Monday night's contest, according to James Palmer of the NFL Network. Head coach Kyle Shanahan made the announcement.

“Kyle Shanahan says DT Arik Armstead is out for Monday.”

The 49ers defensive tackle has been struggling with ankle and knee problems through most of this season. His injury is not ideal for San Francisco, as they're facing the Ravens who have one of the most high-powered offenses in the league this season.

Armstead hasn't played since the Philadelphia Eagles game where the 49ers stomped Philly. On the season, he's accumulated 27 total tackles (four for a loss) and five sacks. He is an impact player in the run game, as Armstead often plugs up the running lanes. But San Francisco will need to find a way to stop the run once again with their star defensive tackle sidelined with an injury.

Despite that, the 49ers have a wildly stacked defense. Nick Bosa and Chase Young form an insane pass rush that's just difficult to stop. Meanwhile, the secondary has been lock down throughout most of the season as well. So, although Arik Armstead is ruled out due to injury, San Francisco should still compete at a high level.

Make sure to tune into the 49ers-Ravens game on Christmas Day, as both teams are fighting for the one seed in their respective conferences.