Liriano wasn't a factor in the Padres plans for this season, so the injury has no short-term effect on the major league roster, but for a raw prospect who has as much potential as any prospect in the organization but also needed as much experience as any, it's a devastating injury that backs up his timetable to the major leagues.
Liriano finished the 2012 season in Double-A and was likely heading back there to begin the 2013 season in hopes of mastering it as he has done in each of his previous minor league stops. Liriano has struggled in his first stint of every new league he's joined, a trend I examined earlier this summer, but he has also gone on to master each level in his second go-round and the same was expected out of him in Double-A in 2013.
The outfield prospect is a true five-tool player, with his power being the only tool that hasn't fully manifested itself in games. Still, as a 21-year-old who made it to Double-A last season, the Padres were continuing with their plan of moving Liriano slowly through their minor league system in order to fully develop all of his skills. For all of his tools, Liriano is still raw in many aspects of his game, most notably his plate discipline, and the Padres planned to give him as many minor league at-bats as possible before bringing him to the majors.
That will still likely be the plan, but it's timetable will simply be pushed back a full year. Liriano's age works in his favor, and he should return to Double-A next season as 22-year-old, no longer young for the league, but certainly not too old for his competition. The elbow injury shouldn't have a long-term effect on his abilities, but it will push his arrival in the majors back at least one year.