Patriots safety Mike Richardson, who was unable to stop the Colts’ Anthony Gonzalez from reeling in a third-quarter touchdown on Sunday, and New England’s defense have been unable to keep opponents at bay in the red zone this season.

The Providence Journal / Glenn Osmundson

FOXBORO — The Patriots’ offense is a work in progress with Matt Cassel running the show and with the injuries that are piling up at the running back position.

So it doesn’t come as a surprise that New England is ranked 22nd in the NFL in red-zone offense, scoring only 14 touchdowns on 30 possessions inside the 20-yard line.

But what comes as a surprise is that the Patriots rank 30th in red-zone defense, allowing teams to score a touchdown when they get inside the red area 63.6 percent of the time (14-for-22).

That is surprising because the Patriots have allowed an NFL-best 17.24 points per game since the 2003 season, and have given up 18.8 points per game this season (ranked ninth).

Patriots coach Bill Belichick said that the Patriots need to improve in the red area on both sides of the ball, and Sunday’s 18-15 loss to Indianapolis is a prime example.

The Patriots scored only one touchdown on four red-zone tries and allowed the Colts to score touchdowns on both of their red-zone attempts.

“If we made any one of those plays (in the red zone), it probably would have made a difference in the game on either side of the ball in any situation,” Belichick said. “So we just have to keep working harder (and) do a good job on those things.”

Belichick said that for the Patriots to improve their red-zone play on both sides of the ball, they need to coach it better and then execute it better.

“It just comes down to better execution,” Belichick said. “(It’s) an area where there is not much field space. It is harder to throw down there than it is out in the field but it is still something you have to do. It is harder to run down there too because everyone is closer to the line and there is less space to defend.

“We just have to do a better job of getting it in and we have to do a better job of keeping them out. That is execution all the way across the board with whatever plays we are calling — runs, passes, defensively, blitzes, zones, mans. I am confident in Matt [Cassel] throwing the ball. I am confident in us running it. I am confident in us stopping them but we just have to do a better job of it that’s all.”

On Indy’s first possession inside the red zone, Peyton Manning completed a 12-yard pass to Anthony Gonzalez with 2:22 remaining in the first quarter which gave the Colts a 7-0 lead.

On New England’s first possession inside the red zone, Cassel got the Patriots into a second-and-5 on the Colts’ 11, but BenJarvus Green-Ellis was stopped for no gain and Cassel threw an incomplete pass to Benjamin Watson. The Pats sent kicker Stephen Gostkowski out for a 29-yard field goal, which cut Indy’s lead to 7-3.

On their ensuing possession, Cassel led the Patriots on a time-consuming, 13-play, 61-yard drive that got the Patriots to the Colts’ 20, but the Patriots couldn’t punch it in for a touchdown.

A Green-Ellis rush for no gain, followed by a Kevin Faulk pass to Wes Welker for a loss of two yards set up third-and-12 on the 22. Faulk then ran for five yards to get the ball to the Colts’ 17, but the Patriots had to settle for another Gostkowski field goal, this time from 35-yards, which made it 7-6 with 1:31 remaining in the first half.

Green-Ellis gave the Patriots their only touchdown in the red area at 7:18 of the third quarter on a six-yard run up the middle that put the Patriots ahead, 12-7.

But the Colts answered on their ensuing drive with a nine-yard touchdown pass from Manning to Gonzalez as the Patriots failed to stop the Colts in the red zone again.

Cassel led the Patriots to the Colts’ seven yard line on their next drive, but the Patriots settled for a 25-yard Gostkowski field goal that tied the score at 15-15 with 11:38 remaining in the game.

Cassel had run for a first down on fourth-and-1 moments before the kick, but Belichick called a timeout just before the ball was snapped.

“The little mistakes can kill you in the red zone,” running back Faulk said earlier this season. “You just have to avoid the little mistakes.”

Traditionally, the Patriots have had a lot of success on offense in the red area, ranking as one of the NFL’s top seven teams in that category since 2004.

Belichick believes that Cassel will perform better in the red area in the upcoming weeks.

“I think Matt has continued to improve every week since Week One,” Belichick said. “It doesn’t surprise me. He works hard, he is a very attentive, smart kid who has talent and every opportunity he gets a chance to practice, take reps and play I think he improves. He’s worked hard on it and I think that has shown up in his performance on a consistent basis.”