Nieuwenhuis has a hamstring strain, Ryan Divish of The Seattle Times reports.
At this point, it's still unclear when Nieuwenhuis picked up the issue or how long it will keep him sidelined. It was recently announced that Ben Gamel (oblique) is expected to miss the start of the season, which leaves an opportunity for Nieuwenhuis to potentially break camp with the team in a reserve outfield role if he's able to get back on the field sooner rather than later. Consider him day-to-day for the time being.