The shortest answer to this question is yes, you will need your PR Card to come back to Canada in a smooth and orderly fashion.
Permanent Residents are of course allowed to travel abroad at any time they wish as long as they maintain basic residency requirements to retain their PR status, but part of the basic purpose of the PR Card is to help you back into Canada.
It is understandable, that occasionally, something will happen and you may not be able to get a new PR Card in time for your return to the country. In which case, you will have a couple of different options to choose from depending upon your situation.
The primary difference between what should be done depends on the mode of transportation in which you expect to return to Canada: commercial or private?
Commercial entry is when one enters Canada on some sort of public transportation; this could be a boat, a plane a bus or other mass-transit vehicle.
If you do not have a Permanent Resident Card, or that card is expired, when you intend to return to Canada you will have to apply for a Travel Document at a Canadian Visa Office. Contact the local embassy to find one.
People re-entering Canada in a private vehicle (most often one’s own car, but it can be a boat or airplane) can use their Record of Landing or Confirmation of Permanent Residence documents in lieu of a Permanent Resident card.
All in all, it is better to maintain a current and well taken-care-of Permanent Resident Card because you never know when you may need to travel abroad.