Not definitely. In Rashford's breakthrough first season he scored 8 goals in 18 games, in the following season he scored 11 in 53. It's a common trend for a lot of young players after they get "figured out", they start to develop their game in different ways, struggle to bring it altogether with wider responsibilities, and have a period of apparent regression.
Greenwood looks an exceptional talent but it's entirely possible he'll go through that. He clearly isn't one dimensional but if you limit that one key dimension - creating space for a shot in and around the box - then it's possible he could fall into a slump. He's often invisible in the build up so if you limit his direct goal threat, the criticism won't take long to build.