Yes, Watson is a long term risk.
But as my wife said at lunch today: any player can get hurt at any time. Want an example? Just look at the first week of Preseason action of all the players (some projected starters) out for the year.
However, if it wasn't for the injury risk, Watson wouldn't even have been available. Tier 1 NFL WRs simply don't come onto the market unless there have been some issues. And often they are in their late 20's bordering on 30.
Watson is 24, not expensive this year, has proven himself in actual NFL games to be an elite WR, and the Rams have the environment, the locale, and the HC, and the money to re-sign him. They got him for a relative song (a 2nd round pick? Who gets a young vet elite WR for a 2nd round pick?).
This was a steal, made possible because the Rams had what Buffalo wanted, and because Buffalo WANTS to lose out this year to get the highest 1st round pick to get the QB they need.
I'd give out 2nd rounders all day long for players like this and the odds of him working out for the Rams are far greater than what a rookie player fresh out of college would be.