Aggression isn't a white skill for MCs so whatever training regime you give them it will be depreciated over time. My MCs don't have particularly high aggression % compared to those players who have it as a white skill and are higher than the MCs and don't get yellows/reds on any level of the frequency of those unforunate MCs.
I try to attend all my matches, and when players get yellow cards I leave them on the pitch for a while. If they foul again but get a referee lecture or other let off then I'll sub them off. Occasionally they'll get a quick second yellow and thus red, but it's a small risk and you can't really do anything about that. After about 20 minutes or so I'll sub the yellow card holder anyway, and it's amazing the number of times that later in the match the substitute that has come on will also get a yellow card...
Nouvion FC have just picked up a cheap spare MC from the Transfer Market as cover for the midfield positions. There are two players suspended for the next Cup match the 2nd best MC and a lower-Q DC who has already got a higher-Q replacement on the books but would have made a very useful bench player. In the League match this morning the red-carded MC got injured for 4 days so the MC hazard is getting bigger...