As Caplan notes, Mathews' injury was first listed on Philadelphia's injury report in Week 6. Mathews told reporters he tweaked the injury on his 63-yard touchdown run against the Panthers in Week 7.
The groin injury, along with a concussion he suffered, could help explain why Mathews wasn't so effective down the stretch. Mathews only rushed for 130 yards and two touchdowns on his final 40 carries (3.25 average) of the season. Mathews had rushed for 409 yards and four touchdowns on his first 67 attempts (6.1 average) during the year.
While it's true Mathews was Philadelphia's most efficient back in 2015, he was far from perfect. He obviously failed to shed his injury-prone label and he still struggled with fumbles (3).
The good news for Mathews is that now he has a full offseason to heal and get ready for next year. Depending on who the Eagles hire and what happens with the DeMarco Murray situation, Mathews could be in line for a significant role in 2016.