He is a functional RB. Definitely NFL level. No team in the NFL needs to modify their defensive game plan to account for Singletary. Nor Moss. From a team perspective that is not necessarily a bad thing. NFL teams don't worry too much about the Patriots, Seahawks, Green Bay or Baltimore running backs. Teams that rely on their running attacks much more than the Bills are right now, generally have RBs that are teams need to specifically worry about. Singletary/Moss are very generic NFL RBs.
Also it is far easier to have a high yard per carry on ten carries than it is on 20 or 25, generally speaking.