Injuries to key players are some of the absolute worst things to happen in professional sports. They can derail a team's season before it even gets started. You're a hot team heading into the middle of the season at 6-2, but then your star left tackle goes down for six weeks. How are you going to protect the blind side of your quarterback now?
Years ago, it wasn't uncommon to see a team immediately look elsewhere for replacements, whether it be free agency, the waiver wire, or to their best trade options at the deadline. They had to, because whoever else was on their depth chart to replace the injured player just wasn't going to cut it as a starter to maintain the high level the team is used to receiving from the position for that many games.