That's correct. You're entitled to whatever disability you contracted for through Manulife. So hypothetically, if you made $4,000 a year before you were injured, then Manulife will pay you 75% of that, or $3,000. Now, if your CPP is $1,200, your new payment from Manulife will be $1,800. Taxes, if you're paying any, should be a wash too.
Like I said though, when you hit retirement age, all bets are off.