I think Diaby's been out for that long, hasn't he?
Part of it is training methods, in my opinion.
The other factors are persisting with players who have poor injury records, signing lots of small, weak players and seemingly rushing back those with dodgy injuries.
How many times do you hear of someone returning ahead of schedule, only for them to have a breakdown and get set back several months?
I don't think that's a coincidence.
He also tends to overplay youngsters, throwing them in for sustained periods that they're not used to and haven't built up to.
That leads to problems, which then don't get sorted, as they're thrown back in.