UPDATE: Just received confirmation from the Seattle Sounders that Cillian Sheridan is, indeed, one of the players on trial with the team and he could appear in tonight's Reserve League match against the Vancouver Whitecaps. In case you needed another reason to head out to Starfire, that probably qualifies.
They are called "rumors" for a reason, but the latest name on the transfer market being linked to the Seattle Sounders is Irish forward Cillian Sheridan. The "rumor" was apparently brought to English language media by Paul Buttner, an Irish reporter who tweeted this:
Reports in Sofia suggesting that CSKA's Ireland striker Cillian Sheridan on way to MLS side Seattle Sounders on loan...less than a minute ago via web 
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Paul Buttner
pbuttner
MLS-Rumors then picked it up and claimed that there's even a chance that Sheridan is already in town. I have called around a little bit and haven't found out anything that would either lend credence to or quash the rumor. There is a reserve match tonight and I suppose it's possible Sheridan could be one of the rumored trialists who are supposedly suiting up.
Assuming there is some truth to this, it would make some sense. Sheridan is young (22), big (6-foot-5) and has a pretty decent scoring touch (.34 goals per 90 minutes in Europa League and the top division Bulgarian league this year). If it's really more of a loan deal, it's doubtful he'd count as a Designated Player, which could still allow for another bigger-name signing.