You can only replace an injured batsman with another who has suffered a similar injury in the past - eg: if a batsman breaks a finger, everyone in the dressing room who has also had a broken finger in their career is eligible to take their place. If nobody has broken a finger, they then look at who has broken a hand, then it moves to arm, and so on. The line becomes blurred with less obvious physical problems, like illnesses. A fiercely debated question in these circles has been, and still is, can you replace a player who has pneumonia with one who simply has man flu? The jury is still out, but the ICC is expected to rule on this by mid- two thousand and twelvty seven.
In an interesting case of these rules being pushed to the limit, in the 2005 England frequently subbed better batsmen in for their tail by squirting a syringe full of fox semen up the noses of both the ingoing batsman and the expected replacement to make it look like they had come down with man flu. Understandably, the Australian dressing room were extremely pissed off about this, while the English dressing room featured some very happy looking foxes.
A protest was lodged by the Australian team post-series, and after a short deliberation the ICC came back with a verdict that simply stated the English were 'a pack of cheating ****s'.