Last season's stats don't mean much to be honest. My DE had a huge jump in production last season then a huge drop off this season. Same player, same team, roughly same level of competition. The only change is the sim.
This sim changes so much that it's tough to compare season over season stats. It's more relevant to look at how he did versus the rest of the league/team, which you probably can't do at this point.
As for proving himself, if he has decent play time, it should be obvious who is more productive. You just need to look at his numbers of plays versus the starter's and figure out how many hurries/sacks/FF/etc the two average per play.
He has 345 plays with 5 hurries, 0 sacks.
The starter has 594 plays with 33 hurries, 8 sacks.
Seems pretty clear to me that the starter performs better. Even if you give this guy another 200 plays, you'd only expect another 3 hurries out of him and maybe 1 sack.