Cut roast in half; transfer to a 5-qt. slow cooker. Combine soup and soup mix; stir in celery. Pour over meat. Cover and cook on low for 8-10 hours or until meat is tender, adding mushrooms during the last hour of cooking., Remove meat from slow cooker. Skim fat from cooking juices. When cool enough to handle, shred meat with two forks and return to slow cooker; heat through. Spoon 1/2 cup onto each roll.