Arizona Cardinals running back David Johnson has been unable to practice this week due to an undisclosed injury.
Johnson was a spectator on the sideline during Thursday's offseason team activities, according to the Arizona Republic.
However, first-year head coach Steve Wilks declined to specify the injury to Johnson, who missed nearly the entire 2017 season after suffering a dislocated wrist in Week 1.
"It's the offseason, and I'm not going to really indulge in injuries or things like that," Wilks told the newspaper. "We're going to talk about the guys that are out there."
Any injury to Johnson is a concern for the Cardinals. Much like was expected last season, he will be the focal point of the offense in 2018.
A third-round draft choice out of Northern Iowa in 2015, Johnson burst onto the NFL scene by joining Gayle Sayers as the only rookies with at least four rushing and four receiving touchdowns along with a kick return for a scorer.
Entrenched as the starter in 2016, Johnson rushed for 1,239 yards and amassed 16 touchdowns. He also caught 80 passes for 879 yards and four scores at Arizona had the league's sixth-highest scoring offense (26.1 points).
However, the injury in the 2017 season opener revealed Johnson's impact on the offense -- the Cardinals averaged nearly eight points fewer than in 2016.
Wilks was impressed with how Johnson looked at OTAs last month. Johnson already has placed high expectations on himself, telling the NFL Network his goal is to become just the third player in NFL history to gain 1,000 yards rushing and receiving in the same season.
The Cardinals will hold a three-day minicamp next week.