This season the Boston Celtics have the opportunity to accomplish a feat that few championship squads have been unable to do – win back-to-back NBA titles. Those who have done it know the magnitude of that challenge.

“We had three chances to defend our title,” said San Antonio Spurs guard Tony Parker, “and it's tough.”

Defending a championship is a learning process for the Celtics. With the recent trade of Sam Cassell, none of the current Celtics have been in this situation before. But an early loss to the Indiana Pacers in November made them aware of the targets on their backs. Now they will have to fend off the attacks in the last half of the season.

“It's not going to be easier,” said Ray Allen. “I think the teams now, we start to step into preparation mode where games get a little tougher, more intense.”

The Celtics have 27 games left this season to vie for homecourt advantage in preparation for the playoffs. Their schedule includes 14 games away from the Garden, starting with a West Coast road trip immediately following the All-Star Break.

From the looks of their schedule, the worst is behind them. They already finished their regular season matchups against the Los Angeles Lakers, will face the shorthanded Orlando Magic twice and nearly half of their remaining contests are against sub-.500 teams. Nonetheless, the Celtics have to avoid being a statement game for playoff-hungry squads.

“You've got to have your guards up. Every team's going out giving you their best,” said Kendrick Perkins. “The biggest thing is just jumping out early in the game. Making sure you come out early and don't give them hope or give them a chance to think that they can play with you.”

Perkins is exactly right, according to Spurs guard Bruce Bowen. Like Parker, he won three championships in San Antonio. He learned to be even more alert in the second half of the season.

“It's a situation where you really have to focus on people more than ever,” Bowen said. “You just understand that you're getting everyone's best game. Wherever you go, it's going to their best game because they realize you’re the champion. So that being the byproduct of things, you understand that that's going to happen.”

The Celtics look to carry their momentum of a recent 8-2 stretch into the second half. That doesn't mean they have to win every single game, though. Prior to being traded to the Sacramento Kings, Cassell explained that the Celtics are not expected to go undefeated after the break. After all, he won back-to-back titles with a 47-35 Houston Rockets team.

“You really don't, you don't keep [that momentum] the entire season,” he said. “There are only certain teams, like the [Chicago] Bulls, they won their second championship and they won 72 games and they kept it the entire season. So it's a marathon. It's a marathon and throughout the course of the season you have ups and downs, you have injuries, guys get sick, you have all types of things. But you find a way to put all that aside and play the game of basketball.”

Veteran guard Derek Fisher won three consecutive championships with the Los Angeles Lakers. He says the Celtics have already survived the stretch between the holidays and All-Star Weekend, which he calls “the toughest months and toughest weeks to mentally keep yourself in a place where you can stay focused.” The Celtics bounced back from a 2-9 slump after a Christmas Day loss to the Lakers and headed into the break winning 14 out of their last 16 games.

“I think the middle part of the season is the tougher part,” he said. “I think at the beginning of the season you're still very excited about what you accomplished the season before. You're still able to ride that momentum and those positive experiences. After the All-Star break and the second half of the season, you see the light at the end of the tunnel as far as having the opportunity to do it again. So you get energized and motivated by that as well.”

Whether they are defending the championship or vying for another title, the Celtics will continue to be second-half targets for the foreseeable future. As long as they are winning, there will be eager teams looking to beat them as motivation for a playoff run.

“It's always difficult when you defend the title,” Spurs guard Manu Ginobili said in Spanish. “You know your opponents want to play hard against you, they want to beat you, they want to prove themselves. But it's not something that only happens to the champions. It happens to other teams like the Lakers, the Spurs, the Suns, the Mavericks, so it's a little harder for all of those teams because the smaller teams want to beat them.” Jessica Camerato is a regular contributor to WEEI.com. She can be reached at jcamerato@weei.com.