How does game do replacements

My leadoff hitter is injured for one more game. His normal replacement bats third. That guys replacement will bat much lower. Who will bat leadoff? The normal 3 hitter or the guy coming off bench swapping positions with the 3 hitter?

Wondering because the lineup change will effect next two games

Actually, neither.
Let's say your 5 hitter is injured. The computer manager (we call him Hal) will keep the first four spots of your batting order the way you want it... but from the 5 spot on down, Hal will do what he thinks is best (which is often a bit of a mess!)
Unfortunately for you, it's your leadoff hitter who's injured. So Hal is going to scramble your entire order, and you might not like the results.
If you're picky about your leadoff hitters and such, like I am, then it's a bummer... but it generally doesn't happen that often. I've learned to live with it.

Jjii66

Actually, I checked. The game out the guy from bench in leadoff spot and left the 3 hitter in his spot, even though he played the leadoff guys spot

Confirmed this in the next game. Thought about it after I posted question