ST. LOUIS — Cooper Helfet’s first NFL start might have been the best debut a Seahawks No. 3 tight end has ever had.

Not too many Plan Cs can make the play the third-year veteran from Duke made in the fourth quarter Sunday. Even fewer former lacrosse players can.

Helfet helped Seattle rally from 18 points down to within 21-19 of the St. Louis Rams with just over 9 minutes left.

The Seahawks faced a first and 14 at the Rams 19. Wilson spread his formation and sent the 6-foot-3 Helfet where he had been most of the game, on the outside against shorter defensive backs. Pressured from the umpteenth time on a drop back this season, Wilson nodded on the run toward Helfet to break off his out route into a sprint down the sideline. The quarterback then sent a strike toward Helfet’s back shoulder.

With a defender leaning on his left, inside arm, Helfet leaned right, into the boundary. He twisted his body back toward the ball and leaped. Wilson exquisitely threw it where only Helfet could catch it, and he did for the score.

“Russell put a perfect ball out there and made it easy on me,” Helfet said.

Wilson’s subsequent two-point conversion pass failed, keeping Seattle behind before it eventually lost for the second consecutive week, 28-26.

Helfet was starting in his fourth NFL game because starter Zach Miller is still recovering from ankle surgery and No. 2 tight end Luke Willson missed Sunday’s game with a groin injury.

The fact Wilson would target a covered Helfet on an improvisational route shows how much faith the quarterback has in the guy with whom Wilson entered the league in 2012. That was when Helfet was an undrafted free agent on his way to being waived injured and then re-signed by the Seahawks.

He originally entered college on a lacrosse scholarship to Johns Hopkins in Baltimore. He went home to Santa Rosa Junior College near his hometown of Kentfield, California, for two years before he transferred to Duke.

Now he might be a fixture in Seattle’s passing game for a while. He finished with three catches for 61 yards Sunday.

“Cooper Helfet, for his ability to step in,” Wilson said, shaking his head after the game. “We’re down Zach Miller. We’re down Luke Willson. He steps in and plays a tremendous football game. It shows the depth that we have at all positions, for the most part.

“And that’s the exciting part.”

Simon’s rough start

Depth is getting severely tested in the defensive secondary. And the results weren’t pretty early Sunday.

Second-year veteran Tharold Simon made his first NFL start in his first league game because cornerback Byron Maxwell was out with a strained calf. Simon had two major penalties in the first 15-plus minutes: a pass-interference foul that extended a Rams drive and a personal foul for grabbing the facemask of wide receiver Brian Quick away from the ball on a running play to begin the second quarter. That helped set up St. Louis’ second touchdown and 14-3 lead.

Simon could have had a third flag, but he got away with shoving Quick in the chest following a play. Quick retaliated and got flagged instead. Simon had penalties for temper-related fouls after plays in the exhibition season, too.

Simon, who last week returned to practice for the first time since knee surgery in early September, then left with what head coach Pete Carroll said after the game was a sprained ankle. DeShawn Shead replaced him as the third option at right cornerback. The first half ended with nickel back Marcus Burley out there for Simon.

Given how long they waited for Simon’s debut before he got hurt again and how costly his lack of discipline was, the Seahawks may be shopping for cornerback help again before next weekend’s game at Carolina.

Yes, rookie WRs exist

Another effect of the Percy Harvin trade: Wide receivers Paul Richardson and Kevin Norwood finally got to play.

After weeks of folks clamoring for the rookie draft picks to be active for games, they made their debuts on Seattle’s opening drive. Richardson was split out wide right on the third and 5 before Wilson threw incomplete out of the back of the end zone. The Seahawks settled for Steven Hauschka’s 24-yard field goal.

Richardson, the team’s top draft choice in the second round this spring out of Colorado, also made a diving catch of Wilson’s pass for a 10-yard gain. It was his second NFL reception.

Richardson finished with four catches in five targets for 33 yards. Norwood had one catch for four yards.

Hurtin’

As expected, four starters were out injured Sunday. Then fullback Derrick Coleman broke his foot with what Carroll said was an awkward step on the turf in pregame drills. That forced backup tailback Robert Turbin into his first fullback duty.

RaShuan Miller was the No. 2 tight end, a day after Seattle signed him off the practice squad and placed rookie defensive end Cassius Marsh on injured reserve because of his broken foot.

The rest of the inactives were expected. Bobby Wagner (turf toe) missed the first of what may be multiple games. K.J. Wright moved from outside to start at middle linebacker, with Bruce Irvin and Malcolm Smith starting at outside linebackers.

Smith and Wright finished with 10 tackles each.

Center Max Unger (sprained foot) was out again, so Stephen Schilling got his second consecutive and second career start there. Schilling then injured what appeared to be his leg and left for a few plays, meaning recently signed Patrick Lewis entered at center. Schilling returned but looked hobbled.

Defensive tackle Jordan Hill (sprained ankle) and offensive tackle Andrew McDonald (last man on offensive-line rotation) were the other inactives.

Lockette takes blame

Outside gunner Ricardo Lockette took responsibility for not noticing the trick punt return in the second quarter on which all Rams except Stedman Bailey went to the left. All Seahawks followed them, instead of the flight of Jon Ryan’s punt to the suddenly lonely Bailey.

Bailey ran 90 yards for a touchdown that made it 21-3 Rams.

“It’s my fault,” Lockette said.

Carroll said Lockette got a late break off the line outside because he was checking with the official there to get lined up correctly, then lost track of the ball and the play. The coach believes if his special-teams ace had gotten set as usual he would have detected the trick and tackled Bailey instead of following the pack of Rams.

Faked out

St. Louis coach Jeff Fisher installed the fake-punt pass play that sealed his team’s win with 3 minutes left Sunday on Wednesday night, after seeing a weakness to that right side of the Seahawks’ punt-return unit up front.

The fake began at the Rams’ 18. If it had failed, Seattle likely would have won the game 29-28 with a chip-shot field goal.

“This is the last thing anybody expected,” Fisher said. “That’s why sometimes those things work.”

Running QB returns

Part of the reason for the Seahawks’ comeback: Wilson returned to scrambling when pass protection broke down in the second half, after a game and a half of not doing it.

That led to four conversions in five third downs after halftime, and six of 12 overall. It was Seattle’s season-best 50-percent conversion rate. The Seahawks began the day 23rd in the league converting 38 percent of third downs.

Wilson thought the rally from 21-3 down and the fight the Seahawks showed despite all that was stacked against them is a positive within defeat.

“I feel a lot of success coming around the corner,” the QB said.