The Mystery Card format is an innovation in simulation baseball that opens up new spectrums of strategy and analysis. In a Mystery Card league, for each player you have on your team, you get one card out of his 5 possible pre-selected years... but you never find out which one.
For example, in the Back to the '90s set, Rickey Henderson's 5 years are 1990, 1991, 1992, 1993, and 1996. You might get his MVP year, or you might get a sub-par year. The only clue you have is how he hits for your team. If he's off to a slow start, is it because you've got his bad card? Or is it just a string of unlucky rolls on a good card? Would you dare cut him? Or maybe you can get something for him in a trade, from another manager who thinks he's going to start hammering? It's maddening. It's addictive. It's baseball.
Assessing player talent
Each player in a Mystery Card player set has a list of 5 years selected from his career - 1 of which will dictate how he performs in your league. For example, in the Back to the '90s set, Rickey Henderson's 5 years are 1990, 1991, 1992, 1993, and 1996. In your league, Rickey will perform based on the stats of 1 of those years at random. Every at-bat he has will be based on the statistical probabilities he produced in that year (e.g. if the year was 1992, he'll hit like a .283 hitter). The specific year will be determined at random at the beginning of your league's season, and will not change no matter where he goes within the league. And here's the kicker - you'll never be explicitly told which year it is. Your only clue is how he hits for your team.
Although the hitting and pitching probabilities will vary for each year for each player, all other ratings are the same for all 5 years. For example, although Henderson hit .325 in 1990, .268 in 1991, etc., and his probabilities for getting base hits and homeruns will vary depending on which year your league has, his fielding, stealing, baserunning and bunting ratings are always the same. Similarly, although Kevin Brown will strike out more hitters if his year were 1998 as opposed to 1993, his endurance, hold, balk and wild pitch ratings are constant and known.
On all lists of players (e.g. draft lists, free agents lists, rosters), the single stat line that is listed for each player is his AVERAGE over his 5 selected years. To see the stat lines for each of his 5 individual years, click on his name. His popup card will always show all 5 possible years throughout the season because, of course, his "true" year is never explicitly revealed.
Ways to tell which card you have
Although you are never explicitly told which of a given player's 5 cards you have (and all die rolls are hidden from play-by-play reports), there is at least one popular method of figuring it out that most Mystery Card managers learn quickly. If you see a position player suffer an injury, you can note the play type that accompanied it (e.g. strikeout), and look through his 5 possible cards to find a corresponding die roll result (e.g. strikeout + injury). If only 1 of the cards has that result, then you know it's that card; if it's on more than 1 card, then you should be able to narrow it down at least. This method can also be applied to players on other teams, of course.
Additionally, you can keep your eye out for other rare events to give you clues - most specifically, triples. However, be forewarned that due to the "More baserunning decisions" Maximum Rule, some triples are actually doubles stretched into triples.
Will I ever find out for sure what card every player has?
At the end of the Playoffs, when a Champion has been crowned, all roster and free agent displays will include the years next to all players' names.
Differences in Pre-Game Lineup Substitution Logic
For all new Mystery Card leagues that are not drafted as of Wednesday, July 31 at 11:01pm ET, the following changes to how the game engine handles pre-game lineup substitutions will take effect.
Selecting players from the bench
In the event that the usual Lineup Depth Chart logic cannot determine a specific player to start at a given position, OR the position is left blank in your lineup preferences, the game engine will now choose a substitution from the bench based on highest salary. Previously, such a substitution would be made based on overall card value (offense + defense), but it's been long pointed out that that method gives clues as to which of the player's 5 Mystery Cards you have.
Previously, in the event of substitutions, the game engine would re-order the lineup in general offense value, which also would give clues towards the identity of your players' Mystery Cards. This re-ordering will not happen anymore for new Mystery Card leagues.
Instead, the game engine will first take all healthy starters in your lineup preferences, plus defensive positions where the usual Lineup Depth Chart logic found intended backups for any injured starters, and push them to the top of the lineup, preserving their relative order. Then, for any defensive positions in the lineup that the game engine had to choose a substitution based on salary, the game engine will place them behind the first group of players, in the following specific position order:
CF, 2B, DH (if any), 1B, RF, 3B, LF, C, SS, P (if any)
- If you leave your lineup completely blank, the game engine will select a full lineup with the highest combined salary, and order them exactly as above: CF, 2B, DH (if any), etc.
- If you have a full lineup set, but your 1B (batting 5th) is injured, but your listed backup at 1B is healthy, the backup will start at 1B and bat in the same slot in the lineup: 5th.
- If you have a full lineup set, but your 1B, and all his Lineup Depth Chart backups, are injured, the game engine will choose a 1B from your bench based on salary, and put him last in the lineup (in front of the Pitcher, if applicable).
- You set your lineup to be 3. Mays (CF), 6. Mathews (3B), 8. McCovey (1B), and all other slots/positions blank. Mathews is injured for the game, but you have Sal Bando listed as your first backup 3B. In this case, the game engine would move Mays (CF), Bando (3B), McCovey (1B) to 1-2-3, and then the rest of the lineup behind them will be 2B, DH (if any), RF, LF, C, SS, P (if any).
- This change does not apply to non-Mystery Card sets (ATG, 2012, 1999, etc.), nor does it apply to existing Mystery Card leagues that have drafted before Wednesday, July 31 at 11:01pm ET.
- This change does not apply to substitutions in the middle of a game.
Other differences from the standard game
The default free agent penalty for Mystery Card player sets is 5/10/20% (5% for games 1-41, 10% for games 42-80, 20% for games 81+), as opposed to the straight 20% it is for other player sets.
The cheapest available player has a $750,000 salary, up from $500,000 in standard player sets.