Doubt it's a few months... Very likely a knee cartilage issue which depending on exactly what they do, should only be a few weeks. If they attempt to repair instead of remove, it would be over a month.
This may very well be the explanation for his somewhat dismal outings this year so far (ie see also Wolskis groin injury definitely has been clearly hampering him as well thus far). He looked real slow on the skates. But try playing a professional sport at 80-90% or lower physically. It just ainīt gonna work.
Hopefully he gets fixed up and is good to go at 100% the second half of the year.
Rupp is not an enforcer by any means. He is a big, physical guy that can play hockey and is well liked in the league amongst GMs - hence the 1,5M pricetag he got. He fights once in a while - with heavyweights too (usually ending in a loss) - but does not fit the bill as a goon. Hopefully we will see the real Mike Rupp when his knee is back to normal - he can be quite an asset on the 4th line...