It becomes a nasty circle, somewhat like a toilet flushing.
E-Lo is on the mound and you *know* you have a chance to win. You're pumped and ready. You play just a bit better and go home thinking I knew it!
Mark gets on the mound and you're either
1. Pressing to help this guy because you really want to help him
or
2. Thinking WTF is going to bite us in the ass tonight
Someone makes an error and you think "damn, I knew it was going to happen"
Players play better when they have confidence. Looking up to see Mark on the mound probably doesn't instill much confidence in this team. Of course that is my guess, no insider information.