You can be sure that during the season several players will be brought up from the minors who will make significant contributions to their team. Last year, Jay Payton was not highly regarded by anyone. Years of injuries had led everyone but the Mets to give up on him. The key is, spend some time. Do some research.
Every team has several prospects that have a chance to make an impact.
The trick is to watch the spring training boxscores. Aubrey Huff is a highly touted third base prospect for Tampa Bay. He could be a solid third baseman for most major league clubs. But Tampa has spent a fortune locking up Vinny Castilla to a long term contract. Barring injury, Huff may not get a chance at Tampa regardless of how well he plays in the spring.