Lowrie, 2005 Red Sox draft choice, regressed in 2011 after hitting .287 with nine homers and 24 RBI over 55 games in 2010. He managed only six homers in 88 games this season.
The 27-year-old has a .252 average with 19 homers and 117 RBI over his four-year career with the Red Sox.
Weiland, 25, spent most of last season at Class AAA Pawtucket, going 8-10 with a 3.58 ERA in 24 starts.
Melancon is likely to become a leading candidate to succeed Jonathan Papelbon as Boston's closer.
The former New York Yankee converted 20-of-25 save opportunities in 71 appearances for the Astros in 2011. He compiled a 2.78 ERA, struck out 66 and walked 26 in 74 1/3 innings.
Papelbon signed a four-year deal in November with the Philadelphia Phillies.