July 31 is often referred to in baseball circles as simply "the trade deadline," but the more accurate term is "the non waiver trade deadline." After July 31, baseball clubs can still make trades. Plenty of August deals happen every season, and the Tigers have been active in August in recent years.
2006: Acquired infielder Neifi Perez from the Chicago Cubs for minor league catcher Chris Robinson
2008: Signed pitcher Freddy Garcia to a minor league contract and later called him up to Detroit.
2009: Aubrey Huff was acquired in August
2011: Delmon Young was acquired from the Minnesota Twins
2012: Acquired utility man Jeff Baker from the Cubs
The selection of available players to trade for is not as great in August as it is in July. The main difference is that after July 31, a player must either clear major league waivers, or be traded to the club who puts in the priority waiver claim before his contract can be moved.
Here is how it works:
Starting Aug. 1, clubs can put players on "special waivers," also known as "major league waivers," "trade waivers," or "recallable waivers." Any player who could possibly be traded can be put on waivers and if they’re claimed, the club can call them back without losing the player.
If a player is not claimed within the waiver period, he can be traded to any MLB team during the month of August. If multiple teams put in claims on the same player, only one club can have the priority waiver claim. Priority is given to the team with the lowest winning percentage in the same league, from worst to first, and then from worst to first in the other league.
There is a protocol whereby clubs generally do not put in a waiver claim unless they are genuinely interested in a player, or in a few rare cases, they want to block a rival team from being able to acquire the player. However, the club that puts in a claim should be prepared to take on the full contract of the player, because that player’s current club can just let the player go if they don't recall him off waivers.
A club that is not a contender has no reason to block another club from acquiring a player, but a non contender might want to put in a claim on a player who is under contract in future seasons. Kenny Williams, when he was GM of the White Sox, claimed Alex Rios from the Blue Jays, and Toronto just let him go. Chicago took on the whole contract.
The Red Sox and Yankees, in particular, are known to play the blocking game, but that can often impact other clubs, and not just the two of them. The Yankees currently hold priority over the Tigers, so if they put in a claim on any players, Detroit is not getting that player, period. The Royals and Indians also have priority over Detroit, so I wouldn't expect many good, reasonably priced players getting through.
The list of players who are placed on waivers every August is quite an impressive list, since there is no risk to putting a player on recallable waivers. Clubs will often put almost their entire roster on waivers, hoping that the player(s) they might trade will sneak through with the pack. The vast majority of players clear waivers. Clubs don’t want to put in claims, disrupting the plans of another club, for fear of retaliation either that season or in another season when they’d like to get a player through. The protocol is "you let this guy through for me and I’ll let that guy through for you".
MLB does not publish the list of players who are placed on waivers. Quite often, information leaks out and sometimes that information is intentionally leaked for one reason or another. All clubs have access to the full list of players who are placed on special waivers.
So, what players might be available in August? Well, any player could clear waivers, but the most likely candidates are those who have contracts that their current clubs would like to be rid of. Half of the Philadelphia Phillies' roster, for example. Players on non contending clubs who are due to become free agents after this season are the obvious candidates.
Good players on National League teams are less likely to make it through waivers to the Tigers, who would be in the 28th waiver position as of today. The Tigers have a better shot at players in the American League, although the Royals, Indians and Yankees would have the priority waiver claim. Waiver claims can not be traded, so teams at the top of the pecking order can’t have teams lower down the list put in a claim for them. The player can’t then be moved to another team without going through waivers again.
One caveat is that a player can only be recalled once during the month. If a team puts a player on waivers, and a claim is made and they recall the player, the next time that they put the player on special waivers, they can’t recall him.
After Aug. 31, all waivers expire, a new waiver period begins, and any player must again clear waivers before they can be traded. However, players who are traded in September are not eligible for postseason play. (Remember Matt Stairs in 2006?) So if you want a player for your playoff roster, you've got to get a deal done this month.