In 2011 Nick McDonald came in when (if I recall correctly) Dan Connolly and then Logan Mankins were injured late in the season, and played very well. The Pats had four players start at center during the course of that season: Dan Koppen, Connolly, McDonald and Donald Thomas.
At that time I thought McDonald was a better prospect than Thomas, but McDonald was injured early in camp last year and never seemed to fully recover; Thomas seized the opportunity and turned it into a nice pay day for himself. Fast forward to 2013 and it was deja vu, with McDonald on the sideline once again.
Hopefully he can get healthy and get his career back on track. For right now rookie UDFAs Matt Stankiewitch, Josh Kline, and McDonald's brother Chris are all better alternatives; at least you can count on them to be on the field.