"Lewis took a knock to his knee at the weekend, which is slightly more serious than first thought. It is frustrating for us to lose him at this stage in the season, but it does mean that other players in the squad get the opportunity to come on in his place and show us what they can do," said head coach Steve Meehan.
In more positive news Olly Barkley, Jack Cuthbert and forwards coach, Martin Haag, have all extended their contracts with the club.
Barkley and Cutherbert have both agreed terms for two more years, and Martin has added another year to his contract.
"We are very happy that both Olly and Jack are staying with us for a further two years," added Meehan.
"They have a lot to offer us in the coming seasons, and play an important part in the squad. They are good blokes to have around the place, both on and off the field.
"Olly is an established favourite at the Rec, and has been playing some great rugby for us over recent seasons. Likewise, Jack has been in great form, and we have seen him make a continued improvement as the season has gone on. His hard work is paying off, and this hasn't gone unnoticed, either by us here at the Club, our supporters, or the Scotland selectors."