If they do get another RB Smith would be the best (cheapest/temporary) choice. Unless they want a more permanent #2, this wouldn't be all bad. He's a free agent, would play cheap, and has experience. If I recall, he doesn't fumble and knows how to block. On the other hand, he's old and doesn't know our offense, so he would be limited on what he could do. Riggs looked OK so far, and both FB's are pretty good, so I'm not sure we need him, but he would be a cheap 4 week investment.
If they are looking for an upgrade at the #2 spot even when Sammy comes back, he's not the answer, not sure if that answer is even out there now, unless Nick makes a substantial trade offer somewhere.