A rumor that English Premier League giants Chelsea plan to make the trip to Seattle this summer has been circulating this morning, and it appears to be legitimate. Jeremiah Oshan of SB Nation Soccer and Sounder at Heart has confirmed that the club will be playing in Seattle, but that confirmation comes with a caveat; Chelsea's opponent for the match has yet to be finalized. That leads to two important questions; who that potential non-Sounders opponent would be and whether a game not involving Seattle would be the friendly included in Sounders season ticket packages.
To be clear, any speculation that the Sounders aren't going to be involved is just that; including a game not involving the Sounders in the season ticket package would be a major shift for the team, and one that would not likely go over with at least some protest. Though a friendly involving two major European clubs would likely be a decent draw in Seattle under any circumstances, the near-guarantee of a sellout and maximum income promised by inclusion in the Sounders season ticket packages is an attractive incentive for touring clubs. It's also hard to believe any of the participants would be particularly enthused about two separate exhibitions featuring major European clubs in the space of one summer competing in the same market.
In terms of potential opposition, the Sounders still must be considered the most likely candidate. As noted above, any other scenario would be a dramatic departure from what has been the club's standard procedure since moving to MLS in 2009. But given that the opponent has yet to be finalized and the front office has made recent statements indicating they are aware of the opposition of a large segment of the fanbase to the Sounders being involved in mid-season exhibitions, the possibility of Chelsea squaring off against another European side cannot be dismissed out of hand. This Boston Globe post from January mentions a mini-tournament involving Chelsea, Italian side Roma and Scottish club Celtic. Given the speculative nature of the post and the amount of time that has passed, it's not out of the realm of possibility that Seattle could have been added to the mix. Of the three other teams, Liverpool seems to be the most likely; Celtic has played in Seattle as recently as 2010 in a game that was not especially well attended by Sounders friendly standards (not to mention the club's recent actions relating to the Sounders.) Roma would be less surprising than Celtic, but given Italian soccer's lower profile in the region when compared to England and Liverpool's massive following, it's likely a decidedly less attractive matchup from the perspective of the game's promoters.
Until an official announcement is made, expect the speculation to continue. If any other major European clubs announce plans to tour the US before that announcement, it could quickly spiral out of control. All that is known at this point is that it's a near certainty Chelsea will be in Seattle at some point this summer. Until new information comes to light, almost everything else is just a guess.