In tennis, as with most other sports, injuries are devastating for players even when they are not career threatening. Players lose ranking points, valuable training time, and overall physical and psychological match readiness as they try to recover. Now this is a general rule, and as such there are exceptions. Indeed, if you're Rafael Nadal injuries just mean that you get to explore a career in making ostensibly sexual music videos with Shakira. For Roger Federer things are unfortunately not that positive.
Federer hasn't had to worry about too many injuries over the last few years because of his strategically planned schedule and excellent fitness, but these things may not be enough to keep his road to becoming the greatest to have ever played the game from getting a little tougher.
Let me clarify. Nadal's ranking has dropped because of his injury. As of March 1, Rafa is ranked as the number three player in the world and has many clay court points to defend between now and the French. If Nadal is unable to return in time, his ranking can conceivably drop even further.
What does this mean? Looking at past draws from Roland Garros, if things stay the way there are now (i.e., Federer ranked No. 1 and Nadal ranked No. 3) we can expect a Federer/Nadal semifinals showdown this year. If Nadal's ranking drops further, it is possible for the two to meet in the quarters or the round of 16.
Federer's head to head against Nadal in Roland Garros in the last four years is dismal (he's never won), and his overall clay court head to head is not much better at 9-2. With only a win at Hamburg two years ago in conditions that were notably different than those at Roland Garros, and another at Madrid that came over a fatigued Nadal, we don't have many reasons to think that Federer will be able change his fate against Nadal at Roland Garros.
Sure Fed may face less pressure now that he has one title on the dirt, but since that title did not come from beating Nadal, I think Roger still thinks he has something to achieve. Moreover, Nadal being the hardworking and determined player he is, will come back to Roland Garros expecting to win it. He has also demonstrated that he is capable of rebounding from injuries very quickly so there is little reason to expect that Nadal's quality of play will fall.
With all of this said, it is important to note that there is still a lot of tennis to be played, and if Novak plays poorly and Nadal returns early and is able to return to number two in the world, Federer might not face the risk of an early exit at the French. Then again, things could work out the opposite way and Federer might have some things to think about.