At first I was frustrated, but then I realized--it's early in the season and he has to get Overbay going. Yanking him risks messing with his confidence, and since the Jays still have two more seasons of Overbay beyond 2009 (combined with the fact that any success they have this season might be predicated on a bounce back season from Overbay), it's probably a bad idea to take that chance in the third game of the season.
At the same time, 2008 was the first year in which Overbay really struggled against lefties. In fact, in 07 he actually hit lefties better than he hit righties. Really, his only real problem against LHP throughout his career has been his walk rate; he actually hits them decently enough (.272 BA, and while his power numbers are significantly lowered, he still boasts considerable gap power--his 2B rate is about the same against lefties and righties).
Besides, left handed hitters batted over .300 against Seay last yaer so it was a good opportunity for Overbay to gain a bit of steam.
And all that said, if it were a one or two run game I think Cito makes the change. But the Jays were at two outs, down 5-1... let Overbay try and work things out.