The championship window for the New England Patriots will not be open forever. Quarterback Tom Brady and head coach Bill Belichick are entering the tail ends of their respective careers -- meaning that the time for this Hall of Fame pair to win a fourth championship together is now.

New England was one game away from reaching the Super Bowl last season, suffering an upset at Gillette Stadium at the hands of the eventual champion Baltimore Ravens.

Can the 2013 Patriots win it all? Here are five reasons why this season's Patriots are neither better than nor as dangerous as last season's:

Reason No. 1: Too many changes at receiver

Analysis: New England had the NFL's No. 1 offense and No. 4 passing game in 2012. If it's not broken, why did the Patriots make so many changes? New England opted to let go of starting receivers Wes Welker and Brandon Lloyd, who combined for an astounding 192 receptions, 2,265 yards and 10 touchdowns last season. Rest assured, the Patriots will not get that kind of production from their 2013 replacements. Danny Amendola will start in place of Welker. Amendola doesn’t have anywhere near the same production and durability as Welker -- he missed more games (20) the past two seasons with the St. Louis Rams than he played (12). The Patriots desperately need their new No. 1 receiver to play 16 games, which is a risky proposition. After Amendola, who will be New England's No. 2 and No. 3 receivers is unknown. There will be competition among Donald Jones, Michael Jenkins, Lavelle Hawkins and rookies Josh Boyce and Aaron Dobson to fill those roles. New England's receivers won't strike fear into opposing defenses this year. Look for the Patriots' passing game to take a step back in 2013, particularly on the outside at wide receiver.

Reason No. 2: Too many injury concerns

Analysis: The Patriots are entering the 2013 season with major injury questions. With so many changes at wide receiver, they expect to lean heavily on their two tight ends, Rob Gronkowski and Aaron Hernandez. But should they? Gronkowski is headed for his fourth surgery on his previously broken right arm, and it’s questionable if he’s going to be ready for the start of the regular season. It's also uncertain if Gronkowski's arm will ever be the same and whether this will be a recurring issue. Hernandez also has been out after major shoulder surgery this offseason and aims to return for August training camp. Backup receiver Julian Edelman, meanwhile, just had another procedure on the same foot to which he suffered a season-ending injury in 2012. Edelman hasn’t been able to stay healthy throughout his career.

Tight end Rob Gronkowski might not be ready for the regular season -- and his durability is a major concern for New England whenever he returns. Robert Deutsch/USA TODAY Sports

Reason No. 3: Tom Brady is a year older

Analysis: Patriots fans have become spoiled by the consistent greatness of Brady. He is expected to put up Hall of Fame numbers every year for New England to be successful. But I’ve seen a major problem with New England for the past few seasons. The Patriots rely on Brady too much and too often, and eventually that will start catching up with the team. Brady, who will be 36 in August, is a year older with a year of more wear and tear. Can he match his 4,827 yards and 34 touchdowns of 2012? He’s playing with all new receivers and his two tight ends are recovering from injuries. If Brady cannot match his 2012 numbers, that will be another step back for New England. There is also already talk of Brady “seeing ghosts” in the pocket as he reaches the tail end of his career. The older he gets, the less Brady wants to get hit. Brady was sacked 59 times the past two seasons, which is the highest two-year total since 2002-03.

Reason No. 4: Tougher schedule

Analysis: For the past several years, New England has taken advantage of one of the easiest schedules in the NFL. It won't get that chance this season. The Patriots are going from the 32nd-ranked strength of schedule in 2012 to the 14th-ranked strength of schedule in 2013. The Denver Broncos, Ravens, Atlanta Falcons, Houston Texans, Cincinnati Bengals and Pittsburgh Steelers are on New England’s 2013 slate. It will be harder for the Patriots to surpass last year’s 12-4 record.

Reason No. 5: The AFC East is stronger

Analysis: New England hasn't had much in the way of solid competition in the AFC East lately. No division team beside the Patriots finished with a winning record in 2011 or 2012. That should change this season. Both the Miami Dolphins and Buffalo Bills are improved and will push the Patriots harder than they have in the past. Miami, in particular, has made closing the gap with New England its primary offseason objective. The Dolphins added dynamic receiver Mike Wallace, receiving tight end Dustin Keller, former Pro Bowl cornerback Brent Grimes and starting linebackers Dannell Ellerbe and Philip Wheeler. Miami added plenty of talent in the draft, led by No. 3 overall pick Dion Jordan. Dolphins quarterback Ryan Tannehill also is expected to make the jump in his sophomore season to compete with Brady. The Bills could make waves, too, if they get solid quarterback play from Kevin Kolb or rookie EJ Manuel this year. Overall, the AFC East won’t be the same cupcake division for the Patriots it has been in recent seasons. The Patriots remain the favorites in the AFC East. But one or two additional division losses could make a difference for New England when it comes to trying to get home-field advantage throughout the playoffs.

Final prediction: Health permitting, this year’s Patriots will still make the postseason. But they will not be a favorite to win a Super Bowl like last season. Denver, Houston, the Seattle Seahawks and San Francisco 49ers are stronger, while the Patriots have taken a step or two backward. New England’s defense is improved, but it’s still not good enough to dominate games alone. The Patriots’ success, as usual, will depend on the offense’s ability to match or surpass last year’s production, when New England led the NFL in total offense -- and that won’t happen in 2013. Last year’s Patriots missed a golden opportunity -- playing at home in the AFC title game -- to win another Super Bowl, and the 2013 season will end without New England taking home the Lombardi Trophy.