It's a conundrum we face every fantasy baseball season. Our best decisions will be made from meaty chunks of data, but to be a good player, we have to make educated guesses off limited amounts of data. Wait for proof is a losing strategy in a competitive league. We need to see plausible upside and act, and we need to make decisions long before we know things for sure.
We also should consider the water of fantasy stats, walks and strikeouts. The K/BB ratio for a hitter or pitcher stabilizes fairly quickly, which means it's not too early to start examining those ratios and trying to figure out actionable takeaways.