The five-time Ballon d'Or winner effectively killed off the contest with his strike half an hour in, taking his tally to 25 goals and 12 assists in the Spanish top flight this term.
Athletic defender Nunez has no doubt that Messi is the undisputed number one performer on the planet, while also lavishing praise on Barcelona in general on the back of their routine home victory.
"He is undoubtedly the best player in the world and he shows it every single game," he told reporters after the match. "In the first half they were fantastic and showed why they're league leaders.
"They were significantly superior in the first half but at half time we talked about it and we had to turn the game around, there was no option whatsoever. We haven't scored but we never gave up.
"There were things to correct, we had to press them higher, we didn't want them left to run free and we had to correct that. We had a close look at ourselves and our attitude and how we came to perform. We had to show with this performance what we're made of between now and the end of the season, our season isn't finished yet."
Barca are 11 points clear of second-placed Atletico Madrid, with a trip to Sevilla up next immediately after the international break.