If I'm Pettersson's agent, I'm looking at Marner and Matthews as comparables. Marner had the same points-per-game in his first three seasons as Pettersson, and Matthews was only slightly ahead (a difference of about 4 points over 82 games). And Matthews also had injury questions; like Pettersson, he had only managed to play a full season once.
Of course, these should be the ceiling of his comparables, but in my opinion, Pettersson's agent should be talking at least 10 million per year to buy 2 UFA years, which would be about 900K less than Marner's deal of the same length, adjusted for cap inflation.