Hey,
Due the way how the attack and the defense work in this tactic these positions get tired the most: DR/DCR/MCR/AMC/ML
Usually if these positions start a match with 95-97% Condition then they have only 68-70% Conditions left at 70-75 min of the match.
DR position is the clear leader in that, this position drains its Conditions the most so he is the 1st candidate for substitution every match, this position greatly contribute to the attack and the defense so you always want to have a fresh player at this position.
If I want to "refresh" the defense at the end of match(it happens when I happy with the current result) then I usually substitute DR/DCR/MCR positions and if I want to refers the attack then I substitute DR/AMC or MCR/ML(it happens when I don't happy with the current result).
As you can see I substitute DR position in both cases because as I said it's very important alwasy to have a fresh player at this position due its great contribution to the attack and the defense..
Btw, if you play "Instant Result" then your results takes significant hit because I see that assistant manager very rarely substitute DR position due its constant high rating during every match and that sucks because after 70-75 min of every match DR position just starts to walk at the pitch and your attack and defense take a very significant hit.
It's a great shame that when AI makes a substitution he looks only at the current rating and he never looks at the Conditions and that's very wrong.