Looking at the race Analysis on MotoGP's website, from what I see, Marquez made most of his gains on Rossi in laps 3 and 5. Lap 3 was almost 7/10's faster and Lap 5 was close to 9/10's faster. Marquez' fastest lap was a 2.04.2 and Rossi's a 2.04.4, from lap 9 on, Rossi spent the rest of the race in 2.05 range, with a last lap 2.04.
Certainly Rossi wasn't exactly the same as Marquez, but he was matching his pace for the most part. We don't know how Marquez would have dropped off as he fell on lap 9. I'm fairly sure he would have dropped off as well. Again, had Marquez have stayed on, he would have won based on his lead. But I don't think it was the massive 10+ seconds he had in Argentina. He was lapping close to a full second faster than everyone in Argentina
Also keep in mind that Rossi was in the beginning somewhat in a fight with Crutchlow. I'm sure he knew he was there as well, so had to take some defensive lines. Not his usual faster lines that he would use not having to defend his position.
I agree, not exact lap for lap times, but for the most part in the 8 laps(with the exception of a couple) he was within(or under) the .5 sec a lap, which is close and keeping within striking distance.
Rossi has always ridden better in the past with the carrot in front and seems to do well having a reference up front. Allows his to study where he's fast and make the most of it.