Clark, who was in hoping for a first international cap having returned to the England fold following a 32-week ban which ruled him out of the summer tour, picked up the injury in Northampton's European exit at Glasgow just a few hours before Johnson was stretchered off in Devon.
Johnson suffered a grade three medial collateral ligament tear just a quarter of an hour into Saturday's Heineken Cup defeat to Leinster at Sandy Park and will sit out all five Six Nations games as England look to build on last season's second-place showing.
"The way it was, the way the Leinster player fell on me, it could have been nine months out, so it's much better than it could have been," said Johnson.
"It's pretty much fully ruptured. There's a little bit of the ligament still hanging on but I think, all in all, I'll take that.
"I was thinking it was a long-term injury when it happened. At least this means I'll hopefully be back before the end of the season.
"It can be a week either way, depending on the rehab, but there are many things that could happen on the way back. As a very rough estimate you tend to say ten or 12 weeks, but there's no real way of knowing.
"I'm just thanking my lucky stars at the moment, that it's not going to be as long as I thought. Then hopefully, I can kick on when I get back fit again."
Johnson currently has just five caps to his name but was always likely to be in the running for the blindside berth when England host Scotland at Twickenham a week on Saturday.
The 30-year-old featured in all three summer Tests in South Africa and then played the two opening autumn internationals against Fiji and Australia, before being dropped for the defeat to the Springboks and the historic win over the All Blacks.