Not a joke. It's apparently a recurrence of an injury that bothered Wagner last Spring, though it's (also apparently) not serious. Wagner will be shut down for a few days, maybe up to a week, just to let give his middle finger some rest. An MRI was negative, and the injury is being described as "tendon sheath stiffness".
It's not a terrible injury, but you just hate to see this kind of thing as Spring Training is winding down. The worst thing is for Wagner's finger problem to become a lingering problem as the season unfolds, so it's critical that he (and the Mets) play things a bit conservatively in the short term to ensure Wagner's health (and subsequent availability) in the long term.
- Mets temporarily shut down Wagner (MLB.com)