The 15-time world champion paralleled the pair's relationship as team-mates to his own experiences with the likes of Mike Hailwood and Phil Read, saying it would be 'impossible' for them to remain amicable for the entire season.
Ex-Yamaha boss Masao Furusawa recently warned the pair's renewed relationship could be unmanageable, although Lorenzo insisted that there would be no repeat of the wall which divided the Yamaha garage in 2010.
"Valentino and Jorge have grown up compared to when they were team-mates last time around, but it's impossible for them to do a whole season as friends," Agostini told Gazzetta dello Sport.
"When you have two roosters in the same hen house, one wants to be more of a rooster than the other one.
"I experienced that with Hailwood, Read and also with Bergamonti: the thing you desire most is to beat whoever races with the same bike as yours, because that's where you see who is stronger.
"If you're behind someone with a different bike then the situation is not as negative, because it's an unbalanced comparison."
Agostini's conviction also stems from the fact he believes Rossi will be a title contender next year - although he still makes Lorenzo the favourite.
"Rossi will win again at Yamaha, but it won't be easy," Agostini said.
"I think he can win one or two races per year, but no domination like before: his rivals are younger and have improved."
Asked to name a favourite, Agostini added: "Lorenzo – although if Pedrosa carries on riding like he did at the end of the season he will be tough.
"Valentino will be in the scrap because he will often step on the podium."