MLS player acquisition rules can be quirky. One of those is Allocation Ranking (sometimes called Allocation Order). Seattle used Allocation Ranking, or a trade directly after the use of Allocation, to acquire Kasey Keller (#1), Sammy Ochoa (traded for #3), Eddie Johnson (traded for after Montreal used #1) and Marcus Hahnemann (traded for after Toronto used #3). The full rule as released to the public;
The allocation ranking is the mechanism used to determine which MLS club has first priority to acquire a U.S. National Team player who signs with MLS after playing abroad, or a former MLS player who returns to the League after having gone to a club abroad for a transfer fee. The allocation rankings may also be used in the event two or more clubs file a request for the same player on the same day when the discovery period opens in December. The allocations will be ranked in reverse order of finish for the 2012 season, taking playoff performance into account.
Once the club uses its allocation ranking to acquire a player, it drops to the bottom of the list. A ranking can be traded, provided that part of the compensation received in return is the other club's ranking. At all times, each club is assigned one ranking. The rankings reset at the end of each MLS League season.
Players that teams other than Seattle acquired 2011 to the present through this mechanism were Juan Toja, Luis Robles, Bakary Soumare, Charlie Davies, Jay DeMerit, Kenny Cooper, Benny Feilhaber and Freddy Adu. Davies, though on loan, did require D.C. United to use their ranking. Most of these have substantial contracts, but the Sounders could have about 200k free which for a half season addition after the larger Euro leagues wrap up would be a time that Hanauer & Co could add someone.
There is some age on the roster at right mid (DP Rosales), at center mid (DP Joseph) and at left back (Gonzalez & Burch) as well as all having some salary space that will open for the full 2014 season. It would be unlikely that a player taken through any mechanism could be a non-DP for the latter half of 2013 and a DP in 2014.
With young, starting quality back up players at RM and CM it would seem the most likely place to look at former United States player coming to MLS to resurrect or end their career would be at left back. The list of players that makes most sense is not long and all have question marks about this usage. All would also be of interest to Portland and Toronto (the teams ahead of the Sounders in Allocation Order).
Carlos Bocanegra - the one time Fire great is struggling to get time in the Spanish second division and just lost his spot as the USMNT captain. The Fire do not hold his rights and so if he came back it would be through Allocation Ranking. He was most often a left back with AS Saint-Etienne. At 33 he would be older and more of a stay-at-home defender than Gonzalez is.
Eric Lichaj - young (24) enough that getting EPL Reserves time is not necessarily a bad thing, the one time rising star may have stagnated at Aston Villa. Now primarily a left back the one time Fire PDL player would likely only consider MLS on loan as a way to give his career path a bit of energy after a disappointing season on a struggling side.
DaMarcus Beasley - He's getting time in the attack at Puebla, but has been used as a left back with the USA. Now 30 and back in Klinsmann's picture if his future is as a defender he made need to play in that role regularly for his club. Oddly, also a former Fire player.
Michael Orozco Fiscal - Also at Puebla. Not a former Fire player, in this case was with the Union on loan. They do not hold his rights. He's 27 and didn't do great with his previous time in the League. Primarily a CB now.
There are in fact several others in Mexico who may make some sense for Seattle. If they feel like making less money, getting slightly more playing time and playing for a coach that has the direct ear of Klinsmann they could come to Seattle. It would be unlikely. Right now Liga MX is just better than MLS. Poaching some talent helps change that, but would be hard to do with 300k-ish annual salary offer. Maybe a miracle run to the Club World Cup helps, but not much if at all.
What is known is that a handful of players will come to MLS. Looking at just potential left backs the caliber of player with all their strengths and weaknesses is obvious. Take a look at the past two years' worth of talent acquired this way and it is obvious that even if the Sounders don't get a quality player they should get Allocation Money, draft picks and/or current talent as another team swaps with them for the precious spot near the top of the Ranking.