Mike Wallace was among several receivers who dropped passes for the Steelers during their loss to the Chargers. (US Presswire)

The Pittsburgh Steelers were heavy favorites at home to beat the San Diego Chargers, so it should have been no surprise that they struggled from the outset and eventually lost, 34-24, Sunday afternoon at Heinz Field.

It was the Steelers' third loss against a sub-.500 this season, as they dropped games at Oakland (3-10) and at Tennessee (4-9). They also struggled at home to Kansas City (2-11), but won in overtime. The Chargers improved to 5-8 with the victory against the Steelers, who dropped to 7-6 overall.

The Steelers' path to the AFC playoffs isn't any less treacherous, as they likely approach the final three regular-season games without left offensive guard Willie Colon. The seventh-year NFL veteran re-injured his left knee early against the Chargers and did not return. He missed the previous two games after originally suffering the injury Nov. 18 against the Baltimore Ravens.

Pro Bowl center Maurkice Pouncey moved into Colon's guard spot, and Doug Legursky took over at center for the Steelers. The two played that way while Colon was out the previous week at Baltimore. If Colon can't return to the lineup, it will be the third straight season that Colon could not finish. He suffered a triceps injury in the opener last fall and missed the entire 2010 season after suffering an Achilles injury training during the summer.

"That's a shame about Willie,'' Pouncey said. "I don't know what's going to happen with him, but we have pull together as an O-line, an offense and an entire team. We need to stop this roller-coaster season and get on a roll for the playoffs or our season will end early, too.''

The Steelers could insert rookie No. 1 pick David DeCastro into the starting lineup at a guard spot this week and keep Pouncey at center. Pouncey has only been active for a few weeks after needing surgery to repair a knee injury after a preseason game Aug. 25 at Buffalo.

Offense: D

There wasn't much good that happened for the Steelers on offense, although quarterback Ben Roethlisberger threw for nearly 300 yards and three touchdowns. However, the bulk of his success came after the Chargers had built an insurmountable lead. He also was picked off and was responsible for the lateral that turned into a Chargers touchdown. The wideouts dropped at least a handful of passes, including Mike Wallace, who had three drops with seven catches for 112 yards and two touchdowns. The running backs were so bad that neither Jonathan Dwyer nor Isaac Redman could gain a yard on third- and fourth-and-one situations. Roethlisberger almost out-ran them both. The offensive line couldn't move the Chargers, allowed two sacks and pressure that forced Roethlisberger to scramble often. Previous game's grade: B

Defense: D-

Chargers quarterback Philip Rivers entered the game with 18 TD passes, 15 interceptions, 11 fumbles (five lost) and 36 sacks, but he threw three scoring passes against the Steelers and led an offense that converted 12 of 22 third-down opportunities and their only fourth-down chance. The Steelers defensive front recorded just one sack against a makeshift Chargers offensive line. They forced no turnovers, and the secondary had a tough time without cornerback Ike Taylor. Cortez Allen started for the injured Taylor, and Curtis Brown moved to the outside in nickel situations while Allen played the slot. Brown was so ineffective that he eventually was replaced by Josh Victorian, signed from the practice squad late last week. Both he and Brown were burned for touchdowns, and all three young corners were beaten regularly for first-down catches. Safeties Troy Polamalu and Ryan Clark also were re-injured during the game, and their status is unknown until Tuesday. Previous game's grade: B+

Special Teams: C

While the team's kickers have been solid, the return game has provided little to no production. Punter Drew Butler boomed a 79-yard kick that is the second longest in Steelers history, but he also had a couple short ones that gave the Chargers good field position. Place-kicker Shaun Suisham nailed a 49-yard field goal to remain perfect from inside 50 yards. He is 26-for-27 this season. Antonio Brown has not done much since he returned as the punt-returner, and kickoff return-man Chris Rainey averaged 22.6 yards per return despite a 34-yarder. The coverage units have been decent, but nothing special. This unit could use an overhaul. Previous game's grade: C

Coaching: F

After an improbable win by ageless quarterback Charlie Batch in the previous game at Baltimore, the Steelers should have had at least a little carryover against the Chargers. They also should have been buoyed by the return of Roethlisberger after a three-game injury absence, but the Steelers played mostly uninspired football until a desperate fourth quarter. The Chargers were reeling after going 2-8 in the previous 10 games, and the Steelers should have been fired up. But, as stated earlier, the Steelers continue to stumble during games against team's with losing record -- including three losses now -- and head coach Mike Tomlin and his staff must be held accountable. Tomlin gave a rousing post-game rant in the locker room, but that was too little, too late, for the Steelers this week. Maybe one is needed in his pre-game commentary prior to next week's game. Previous game's grade: B

Follow Dale Grdnic on Twitter for real-time Steelers' updates @CBSSteelers.