tejada has been solid so far, but i don't like his chances of keeping it up. E. Santana has obviously looked pretty bad, but I think he's worth more than tejada for the chance that he turns it around. i've not done 100% diligence on his injury, but it's possible he's just working out the kinks and regaining strength. Owning Santana is like a cheap call option on his return to 2008 form. When reyes comes back, you'll regret having traded for tejada.
I actually think you run a greater risk believing Ervin will turn around and nab better value from him in the long run. His timing is off and so is his velocity. That said, I don't believe Tejada will drive the ball as he has been and his K rate is much lower than career norm (that will catch up). If you had gotten Downs on the side, then I would take that deal because even if ideally Ryan could be worked back into closing, it's awfully tough with Downs being lights-out, meaning I believe he'll stick to closing.
I'd stick it out with Ervin for now and see if he makes progress the next five starts or so, but I'm not too optimistic about his chances this year, given that an MCL strain in the elbow can cause discomfort every now and then.