Hip flexor injury is very common in sports, especially soccer, football, running, hockey, any sport really that demands explosive movements.
While they can be serious, I do think some of these posts are being slightly over dramatic in this case.
I'm well aware of hip flexors, I've had hip issues myself. They certainly can be serious, and when it comes to Daniel Alfredsson at this point in time, there really isn't anything unserious about it. He's had enough hip issues in the past: