Siegrist had been on the DL for two months with a left forearm strain. In five rehab appearances with Double-A Springfield, he allowed no runs in 4 2-3 innings. In his most recent outing Monday, he worked 1 1-3 scoreless innings.
As a rookie last year, Siegrist set a franchise record for a reliever with a 0.45 ERA.
This season, he was 1-1 with a 3.60 ERA in 23 games with 27 strikeouts in 20 innings.
Greenwood was 1-1 with a 4.74 ERA in nine games and worked two perfect innings Tuesday.