What do you mean with weakest part of his game? Is Djokovic's forehand the weakest part of his game despite being impressive?
Nadal is one of the greatest returners of all time. He is the greatest returner ever on clay, and the 15th greatest returner ever on hard courts:
https://www.atptour.com/en/stats/le...r&surface=hard&versusRank=all&formerNo1=false
The Return Rating is calculated by measuring 4 factors:
1) % 1st Serve Return Points Won
2) % 2nd Serve Return Points Won
3) % Return Games Won
4) % Break Points Converted
Even Djokovic himself stated that Nadal is a great returner.
"I will have pression on my serve because Nadal returns well" (7:40)
The Nadal-Djokovic matches are full of mutual breaks!! I have no problem to admit that Djokovic's return is better, but it doesn't mean Nadal's return is not great as well. They have different styles by the way. Djokovic is a more aggressive returner which typically goes for the return winner, while Nadal focuses more in returning a high percentage of serves to later win the point form the baseline.