Lewis is probably a good pick up for deeper leagues or NL only leagues. Not sure about a 12 team 5x5 mixed, unless you have deep rosters.
He is a lefty in a platoon situation, and in deep leagues that is a good bet to get enough at bats since most pitchers are righties. At least as a stop gap while Roberts is out.
He could be better than average. He was listed in Baseball America as #7 in the SF system, and as best athlete. So, he is certainly enough of a prospect that he could stick and do well.
Or he could be hot for a while then fizzle. I would guess a few resonably productive weeks and then sent back down for steady playing time.