The prospect is believed to be outfielder Johermyn Chavez, and the trade will likely be officially announced Wednesday.
"I didn't even know until today who I was traded for," League told MLB.com. "You never see things like this coming, for me, anyway. I'm staying positive. It's a good organization I'm going to. The only downside is that I'll be missing the guys I kind of grew up with through the Minor Leagues."
In League, the Mariners get another hard-throwing right-hander to enter into the late-innings mix in Spring Training alongside last year's setup man, Mark Lowe, and closer, David Aardsma. League, 26, set a career high with 67 appearances in 2009 and struck out 76 batters in 74 2/3 innings. In his career with Toronto, League went 7-10 with a 4.09 ERA in 202 1/3 innings. He can hit up to 97 mph on the radar gun.
"I'm excited because I had [Mariners pitching coach] Rick Adair as Double-A pitching coach [in New Hampshire in 2004], and in talking with [former Mariners and Jays reliever] Miguel Batista, he had nothing but good things to say about the Mariners, the direction they're going," League said. "I've heard nothing but good things about [Seattle manager] Don Wakamatsu. I'm excited to be here, excited that they want me, and I'll be trying to help out as much as I can to win games."
Chavez, a native of Venezuela who will turn 21 on Jan. 26, projects as a big league power prospect at 6-foot-3 and 220 pounds. Chavez hit .283 with 21 home runs, 89 RBIs, 87 runs, a .346/.474/.821 on-base-plus-slugging line and 10 stolen bases while primarily playing right field for low Class A Lansing in 2009.
He earned an R. Howard Webster Award at the end of the season, honoring him as the Player of the Year with Toronto's Lansing affiliate and has hit one homer and driven in four runs in a 13-game stint with the Navegantes del Magellanes of the Venezuelan Winter League.
Morrow, 25, was taken No. 5 overall in the 2006 First-Year Player Draft and shuttled between starting and relieving in several big league stints. Last year, he opened the season as the team's closer, lost the job early in the season and was eventually groomed in Triple-A to permanently be a starter. Morrow resurfaced in Seattle as part of the starting rotation and clicked in his last outing of 2009 when, Morrow, whose fastball has been clocked as high as 99 mph, threw eight one-hit innings against the Oakland A's. Morrow was headed to the Jays' complex in Florida to take his physical Tuesday and was unavailable for comment.
Morrow was 2-4 with a 4.39 ERA in 69 2/3 innings for the Mariners in 2009 and 5-3 with a 3.60 ERA over 10 starts with Triple-A Tacoma. Overall, Morrow has gone 8-12 with a 3.96 ERA in 197 2/3 innings with Seattle. In 15 career starts, he has gone 4-3 with a 4.42 ERA.