In the offseason after the 2007 season, the Sox and Twins were discussing Johan Santana for Jon Lester + Jacoby Ellsbury. That is, if I recall, what the Twins were looking for. At the time, Santana was easily one of the best pitchers in baseball, just 28 years old, lefty (well, he's still lefty, obviously), winner of 2 Cy Young Awards in the previous 4 seasons.
Meanwhile, Lester and Ellsbury were just prospects, really, not having done much in the majors. Each had played for the 2007 WS winning team, but neither played very much during the course of the season. We all saw potential in those two guys, but it was just that - potential.
Just focusing on Lester and Santana, here's what each guy did from 2008-12:
TOTALS
Lester: 161 g, 74-46 (.617), 1018.2 ip, 3.63 era, 123 era+, 1.27 whip, 8.4 k/9
Santana: 109 g, 46-34 (.575), 717.0 ip, 3.18 era, 127 era+, 1.20 whip, 7.6 k/9
AVERAGE SEASON
Lester: 32 g, 15-9, 3.63 era, 204.0 ip, 123 era+, 1.27 whip, 8.4 k/9
Santana: 22 g, 9-7, 143.4 ip, 3.18 era, 127 era+, 1.20 whip, 7.6 k/9
And now Santana is out for the year, and who knows if he'll ever pitch again. Meanwhile, Lester enters his age 29 season, as the #1 starter for the Red Sox.
I loved Santana and really wanted him to pitch for the Red Sox, but that turned out to be a great non-trade for them. Obviously adding in the fact that Ellsbury is a heck of a bonus, the non-trade was even better for Boston.