Daisuke Matsuzaka's progress in returning from Tommy John surgery has been surprisingly free of trouble, and because of that, he's on target to miss the just over the minimum amount of time for those who have the procedure. His surgery was performed on June 10 of 2011, and with a rehab start at Single-A Monday, he'll be back May 18 after five rehab appearances.
Back in what capacity is the question at hand right now, as the Red Sox rotation is currently healthy and five deep, with Aaron Cook waiting in Pawtucket for his chance at the roster. There's nothing wrong with having pitching depth, as it's early yet. Boston's rotation looked like healthy this time last year, too, and it wasn't that way for long, courtesy of people like Matsuzaka.
Neither Cook nor Dice-K is a savior, either, but they're likely both more ready to pitch than what the Red Sox trotted out as replacements in 2011. In fact, Dice-K was equally as unproductive as those pitchers, during a stretch in which his elbow required Tommy John surgery to begin with. Damaged goods Dice-K was the same as healthy Andrew Miller, Kyle Weiland, and Tim Wakefield.
Dice-K as a reliever is potentially problematic given his long warm-up times, but if he's on something of a schedule, as a shadow for Daniel Bard, for instance, then something could be worked out. A tandem starter approach, where Bard goes as long as he's able, and Dice-K comes in to relieve him when his arm is finished for the day, whether that be at 90 or 100 or 110 pitches. If Matsuzaka is going to pitch well now that he's healthy, it's something to consider, especially since it would take a load off of the rest of the bullpen, allowing most of them to be fresh for the other four starters.