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Mattson is a month behind schedule due to shoulder soreness, Roch Kubatko of MASNSports.com reports.
Mattson isn't expected to get on a mound until April and will still have weeks of buildup to do at that point, so he'll almost certainly open the season on the injured list. The 26-year-old made his big-league debut last season but threw just 4.1 innings, so he's not necessarily in line for a significant role once he gets healthy.
