Fresh on the heels of their most recent two game winning streak the Boston Celtics reach the official mid-point of the 2008-2009 NBA season tonight.Nobody ever said that their title defense was going to be easy, and in a season where the green have won 19 in a row, and also managed to lose seven of nine the Celtics have certainly seen their share of ups and downs.Celtics coach Doc Rivers told his players that their most recent tough stretch can only make them stronger.”Sometimes you have to go through something to get something,” he said. “Last year we didn’t have this at all. I was extremely concerned going into the playoffs and I made the comment, my one concern was we hadn’t been tested; we breezed through the season, 66-and-whatever.”Well, we can’t say that this year.”If the Celtics have learned one thing through the first 41 games it is that there are no easy victories. Opposing teams take pride in defeating the world champions and for a team like the Charlotte Bobcats who probably wont even be in the playoff conversation defeating the Celtics is their Game 7.Making things even more difficult are the recent rash of injuries facing the green. Center Kendrick Perkins is the most recent Celtic to go down. For Perk, missing time with a bad shoulder has become an annual right of passage. Perk himself hopes to be back soon, saying “I was told to sit out a week so I guess I’m out until Monday.”It does feel better; it gets better by the day.”Also on the shelf is Tony Allen who is missing time with a sprained ankle. Celtics star Paul Pierce could use a vacation as well.Nursing a bruised knee, the captain aggravated that injury when he ran into teammate Brian Scalibrine. The captain however is not planning any time off.”Actually, I feel pretty good,” said Pierce. “It feels pretty good today. I’ve had a lot of ice on it. It’s like when you sprain it and jolt it again. Hopefully, I don’t hit it again so it can heal. It’s been a nagging injury, but something I can play through.”For Celtics fans looking for a new addition or two for the stretch run, you can scratch another name off your list. Semi-retired free agent Robert “Big Shot Bob” Horry recently told the media that he’s content to stay retired saying “I’m just hanging out with my family. That’s all.”A more realistic option for the Celtics is a guy already sitting on their bench. Veteran Sam Cassell, who has more technical fouls than he does minutes this season, recently said that “he’s ready.” With limited free agent options, and a trade unlikely it may be “Sam I Am” to the rescue sooner rather than later.