Some lurchers do limp for a while after a good cut. The get used to limping for a while and carry on with it after it heals, best thing to do to see if it is glass, is walk the dog on the lead for a few miles, don't let it off the lead, the idea is to get the dog walking properly while it has it mind on other things, like being let off the lead. If it still limps when you get back, then there may be something left in the pad. Try this for a couple of days and see how you go, I wouldn't let the dog off the lead until you know for sure.