UPDATE: The prospect the Mariners are receiving will likely be single-A outfielder Johermyn Chavez. Chavez, 21 in January, is a power-hitting prospect with high strike out rate and lacks plate discipline.
25 year old Morrow posted 4.39 ERA along with 1.58 WHIP, 8.14 K/9 rate in 69.2 innings for the Mariners. His strike out rate is high, but his walk rate sits at 5.68 per 9 innings. He was projected as the Mariners' No. 4 or No. 5 starter in 2010. Morrow was taken fifth overall in the 2006 draft, as the Mariners decided not to pick Tim Lincecum, who the San Francisco Giants later took.
League pitched in 67 games (74.2 innings) for the Blue Jays, posting 4.58 ERA, 1.25 WHIP, along with decent 9.16 K/9 rate. He should take some of the work load from Mark Lowe and Sean White for the Mariners.
A prospect in this deal is yet to be known, but it has to be a solid prospect to balance out the trade. Maybe J.P. Arencibia, the top catching prospect for the Blue Jays? Let's hear it in the comment section.