During Geno Smith's senior year at West Virginia University, the Mountaineers went on a five-game winning streak to open the season. During the 5-0 run, Smith threw for 1,996 yards, 24 touchdowns, 0 interceptions, and a 81.3% completion rate. The offense averaged 52 points per game, and the defense gave up 35 points per game. Life was good.
In the sixth game of the season, the Mountaineers faced the Texas Tech Red Raiders, and the season began to unravel. For the next five games, the Mountaineers lost every matchup. The defense couldn't stop a nosebleed, and was suddenly giving up an average of 49.6 points per game during the losing streak, in contrast to the 35 points previously. Suddenly, Smith had to score points on essentially every drive, and he started to struggle. His footwork fell apart with the pressure to continually put up points; essentially, to put the team entirely on his back. In the two games following this five-game losing run, the defense allowed just 12 points per game, and Smith was able to reassert himself and finish the regular season at 7-5.
The same is happening now, as it happened last season. Smith has to put up points, and he's feeling the pressure to do it entirely on his own. When Eric Decker is out, there is nobody capable of beating one-on-one coverage, with the sometime exception of Jeremy Kerley. Smith is forced to put the team on his back, to use a cliché, and he can't do it alone. When that happens, he crumbles.
It's painfully obvious that Smith will never be an "elite" quarterback in the sense that he'll never be the kind of guy that can single-handedly keep his team alive, like Andrew Luck or whomever you choose as an example. When he's had help, such as a healthy Decker, he's shown some success. In the meantime, and until the New York Jets are willing and able to provide some help, expect things to get worse before they get better.