Wickit, a substitute may be brought on only for an injured fielder. However, a substitute may not bat, bowl or keep wicket. The original player may return if he has recovered. A batsman who becomes unable to run may have a runner, who completes the runs while the batsman continues batting. Alternatively, a batsman may retire hurt, and may return later to resume his innings if he recovers. I have quoted the MCC rulebook verbatim
