Having dropped 23 points adrift of Yamaha team-mate Rossi with a lacklustre performance at Silverstone and a crash at Misano, Lorenzo clawed nine points back by winning at Aragon last weekend when Rossi could only finish third behind a combative Dani Pedrosa.
"It was a really important race. I really felt that if I lost even more points it would be almost impossible to win the world title. It was key," said Lorenzo.
"It was a very important and very difficult and very emotional race, because after these two races where I made some mistakes and probably had some bad luck with the weather, I had some negative feeling inside.
"The victory means I can forget those two races a little and just look forward."
Lorenzo believes he would have been hard pressed to beat Marc Marquez in the race had the reigning champion not crashed while catching him on lap two.
Many of Lorenzo's victories have been sealed by extremely fast early charges to try to break the gap to the pack or force rivals to make mistakes trying to keep up, and he intimated he had tried the same technique to wrong-foot Marquez at Aragon.
"I was quite lucky with Marquez's mistake," said Lorenzo.
"My first or second lap was probably a little bit hard for him to recover all those metres under braking and he pushed too much on the front tyre.
"I knew Marc was very strong, especially after the warm-up when he did the fastest lap on the last lap on an old tyre - I knew he'd be very strong at the end of the race.
"It would have been very, very complicated to beat him."
Lorenzo was also relieved that Pedrosa put up such a strong fight with Rossi for second.
"When I saw Dani was 2.3 seconds behind, I breathed," said Lorenzo.
"But I said to myself I had to keep focused because Dani always has good pace on this track and having Valentino on his back he'll push at the maximum all the time.
"They told me Dani was very brave and they were overtaking five or six times in one lap. I have to thank Dani because these four points are very important for the championship."