Detroit is quite banged up on defense as the IR list shows but they have been pretty healthy on offense. Their ability to score points on offense wins them some games where their defense cannot hold the opposition. Last night is a good example.
Injuries are an excuse, and not just in Dallas. Many talking heads have used injuries as an excuse for San Francisco. Also, every team has injuries, but the volume of injuries is the problem for teams like Dallas, Cleveland and SF.
The Cowboys injuries have been all over the place but started with Sam Williams in the offseason. Their 2 starting CBs have not yet played a game together this year. They have have injuries on the OL, WR, QB, TE, DE, LB, S, and CB. No one has that kind of quality depth to overcomes that much disruption.
But the truth is, the Cowboys were weak on the defensive line and at RB and WR before the injuries. The injuries just made it that much worse. Dallas had to sign Joseph and Watkins at DT at the beginning of the season just to have a rotation. Injuries hurt but they hurt a lot more when the team is unprepared to start the season.