He was really noticeable before he missed a few games with injury (3 games- hamstring). He either is still hurting (probably reasonable expectation with hamstiring) or opponents had the opportunity to gameplan for him better as time went on ... or a combination of both. A rookie DT is generally overmatched moving to the NFL. Flashes of success is really all you can hope for (in a rookie). The first off-season will show if he's learning and training like a pro or relying solely on athletic ability. He's either going to take a big step forward in his second year, or he's going to plateau and be "who he is". I would hope and expect that Leonard Williams' impact will have a big influence on that projection. At this point, the world is his oyster but he has work to do ........
Frankly, Murphy will have a larger impact on the team's success than solely his position. The high draft pick for him last year and the lackluster production on Offense during the season really points toward a higher investment in Offense this off-season. If the interior OL is as high a priority in the Front Office as it is here on .net, the D will likely have to improve without a lot of investment. EJones will (hopefully) be the focus and might be the only big contract on D. (Am I forgetting anyone?) Hopefully there's some luck on late picks or UDFAs but Murhpy's progress and EJones' growth under MMac may need to make up for the relatively poor performing Secondary.
There's only so much money/picks to spread around and the Offense should be getting the vast majority this off-season. IOL(3-5), replace TLockett (1), TE (1), RB pipeline, QB pipeline...