MLB.com's Doug Miller throws together a list of players who could go from non-roster invitees to indispensable cogs in the team's...wheel (right metaphor?).
And Gustavo Chacin makes that list:
In 2005, while with the Blue Jays, Chacin finished fifth in AL Rookie of the Year voting after going 13-9 with a 3.72 ERA in 203 innings at the age of 24. He wore glasses on the mound. He had style. Injuries and eventual shoulder surgery in 2007 slowed him down, but he re-emerged in the Phillies organization in 2009 and was healthy enough to pitch 115 1/3 respectable innings, carving out a 3.20 ERA in 19 starts.