The Lions saw cornerbacks Terrion Arnold and D.J. Reed suffer injuries yesterday. The former does not appear to be in line for an extended absence, but the latter could find himself on injured reserve. 
Arnold suffered a shoulder injury on Sunday, although head coach Dan Campbell said (via Ian Rapoport of NFL Network) he does not anticipate a lengthy spell on the sidelines. That will be welcomed news even with Arnold’s struggles early this season and the resultant plan to lower his playing time. Such a setup may need to be put on hold for now.
Reed was carted off the field with a hamstring injury. Based on Campbell’s post-game remarks, Rapoport notes a move to IR could be in store in this case. That would ensure at least a four-week absence for Reed; it would also leave Detroit without a perimeter starter for an extended period.
During the offseason, the Lions expectedly lost Carlton Davis on the open market. Detroit took the free agent route to bring in Reed on a pact similar in length and value (three years, $48MM) to the one Davis landed with the Patriots. Arnold and fellow returnee Amik Robertson have been key figures in the secondary to date, and that will of course remain the case moving forward. Still, losing Reed for any notable period will mark a blow to Detroit’s secondary.
The 28-year-old has served as a full-time starter with his newest team, collecting four pass deflections early on. Reed also notched his first interception of the season shortly before suffering the injury. Further testing in his case will be worth watching closely, as it will inform the Lions’ decision with respect to an IR move taking place or being avoided.
The plan for reducing Arnold’s usage was aimed at giving Robertson more time on the boundary as opposed to the slot. If Reed winds up missing considerable game action, Robertson may be forced to handle a full-time role on the perimeter opposite Arnold (presuming, of course, he does not wind up being sidelined for an extended period). In any case, defensive adjustments will likely be necessary as the Lions look to build off their strong start.