Excuse a quick interlude: Why do teams continue to trade with the Yankees? Why would the A's -- a team fighting to make the playoffs, a team with two consecutive postseason losses to the Yankees -- help facilitate a three-way trade that allows the Yankees to pick up one of the best pitchers in the American League?

Just curious. Back to our regularly scheduled trade analysis.

Yankees get: Jeff Weaver, give up Ted Lilly, John-Ford Griffin, Jason Arnold
How good is Weaver? Ignore the 6-8 record; he's very, very good. Heading into Saturday, he ranks ninth in the AL in ERA and fifth in innings pitched. He's allowed just four home runs, is young (25), durable (never been on the disabled list), and signed through 2005 at a good price ($4.1 million this year, $6.25 million in 2004 and $9.25 million in 2005).

Of course, not that the Yankees are concerned about payroll.

How will Weaver fit in the Yankee rotation? He could be their best pitcher:

Pitcher               ERA
Orlando Hernandez    2.89
Jeff Weaver          3.18
Andy Pettitte        3.63
David Wells          3.66
Roger Clemens        4.20
Mike Mussina         4.54

(Not listed: Sterling Hitchcock, the $12 million blunder.)

Of course, that's six pitchers, so one will get bumped from the rotation. Moose to the bullpen!

What's scary for the rest of the AL is that Weaver moves from one of the league's worst defenses to one of it's best. Using a stat called Defensive Efficiency Record, which tells the percentage of balls in play that are turned into outs (home runs not included), the Yankees have the third-best defense in the AL while the Tigers have the third-worst. Weaver has allowed a .243 opponents' average this season; expect that to drop with an improved defense behind him.

Is there a potential downside? Yes, last year Weaver threw the second-most pitches in the AL; this season, he's fifth. So he has thrown a lot of pitches at a young age, although we've seen no signs of ill effects because of that.

The Yankees did give up a nice young lefty in Lilly and two good prospects in Griffin and Arnold, but give GM Brian Cashman kudos for this deal. Weaver provides depth for this year and, perhaps more importantly, an anchor for next when David Wells is gone and Roger Clemens a year older.

Tigers get: Carlos Pena, Franklyn German, played to be named, give up Jeff Weaver
Pena, who the A's acquired in the offseason from the Rangers, was expected to be one of baseball's top rookies. But the first baseman hit just .218 and struck out 38 times in 124 at-bats, earning a demotion back to Triple-A, where he was hitting .240 with power (10 HR in 171 ABs) and lots of strikeouts (48). His track record through the minors is very good: he should hit for power and draw walks, but his dip in batting average and high strikeout rate is cause for alarm. He should immediately be installed as Detroit's first baseman.

German is a Double-A closer who ranked as Oakland's ninth-best prospect by Baseball America entering the season. He throws very hard and has fanned 59 in 40.1 innings this season and fanned 93 in 63.1 innings last year. Minor-league closers have a poor track record, so German must be considered a big question mark right now.

It's a risky trade for Detroit. If Pena develops into a 30-homer, 100-walk hitter, than he provides a big bat the Tigers need. But now they need to replace their best starting pitcher.

A's get: Ted Lilly, John-Ford Griffin, Jason Arnold, give up Carlos Pena, player to be named
OK, here's why Oakland made the trade: Lilly has pitched very well this year (3.40 ERA, .202 batting average allowed) and Griffin and Arnold are quality prospects. Considering the struggles of Cory Lidle (2-7, 5.30 ERA), Billy Beane has improved his starting rotation, giving up a player in Pena who wasn't likely to make an impact this season.

Griffin was the Yankees' No. 1 pick last year out of Florida State and he's certainly a Beane-type player: a young hitter with good potential and the willingness to talk a walk. He hit .267 with three homers in the Florida State before a promotion to Double-A, where he's hit .328 with five home in 67 at-bats.

Arnold was the Yankees' No. 2 pick last year out of Central Florida. He blew away the New York-Penn League after getting drafted and was dominating for Class A Tampa (2.48 ERA, 83 K's in 80 IP) before promoted to Double-A (4.15 ERA, 18 K's in 17 IP).

The trade has big high-end potential for Oakland. On the other hand, Lilly still has to prove he's as good as he's pitched ... and Beane has to hope Weaver doesn't beat the A's in the postseason.

David Schoenfield is the baseball editor at ESPN.com.