By Jennifer Piacenti Despite slow starts, Trey Mancini, Jesse Winker and Josh Donaldson still have value. We are just about a month into the fantasy baseball season. There’s a long way to go, but it’s frustrating when your studs aren’t performing like studs. Though there are some players that are already showing cause for concern, here are three guys you should exercise patience with. If you don’t have these guys on your roster, they are also players you should consider trading for while there is still value. Trey Mancini (1B/DH), BALThere were a few concerns about Mancini coming into the seas…