I would guess that they're very concerned with Phil Hughes to sign someone like Silva, whose career numbers (70-70, 1242 ip, 93 ERA+, 1.40 WHIP, 4 K/9, 1.7 BB/9) are less than thrilling.
Silva did, however, have a solid 2010: 10-6, 113 ip, 103 ERA+, 1.27 WHIP, 6.4 K/9, 1.9 BB/9. The question is: how many more turns will Hughes get if he looks the way he did on Friday? And who's higher on the depth chart, Silva or Kevin Millwood?
The fact that Larry Rothschild was Silva's pitching coach in Chicago also motivated the deal.