Hendriks is a reliever - albeit a good one - and therefore is anything but irreplaceable. Good relievers pop up all the time, year-to-year.
There's nothing wrong with acquiring proven, solid back-end rotation depth in November, February, April, July - any time of the year. The trade was fine, and it certainly does not prevent the Jays from making a subsequent, and more significant, addition to their rotation. Depth is a good thing.