Nelson (shoulder) threw 28 pitches in a bullpen session Tuesday, MLB.com reports.
Nelson has now completed two side sessions since landing on the injured list, after the right-hander previously threw a 16-pitch session Saturday. Diamondbacks manager Torey Lovullo said the team is undecided about whether Nelson could return to the starting rotation or be used as a multi-inning reliever once he makes his way back from the IL. With four games left in the regular season, the bullpen is probably a more realistic landing spot for Nelson if he gets activated.