For all my matches (those I watched), there is always a substitution when my players get injured if I still have my 3 or 2 or 1 more substitution available to be made.
The rules of substitution from what I have seem is position for position, in general.
What I meant general is defender for defender even if your center back is injured the next in line is a good side back, he will still get substituted in. However these position - wing back, defensive midfielder, attacking midfielder, winger - can be substituted by two different positions. (not 100% sure for wingers).