The former Lions tourist hobbled out of Bath's Heineken Cup thrashing of Aironi on Saturday and, with England's Six Nations opener against Wales now less than three weeks away, it is news Johnson could have done without.
No official prognosis has been made at this early stage but Johnson will at least be relieved that both Moody and Bath coach Steve Meehan do not believe the setback is overly serious.
"Lewis twisted his knee in a driven lineout," said Meehan after Bath ran in eight tries at The Rec.
"He doesn't seem too concerned - it's maybe a week off. He's quite confident there's nothing sinister there.
"I haven't spoken to our medical team but with these things it's 24 or 48 hours later.
"He will be having treatment tonight and through tomorrow - keeping it iced and elevated."
While Johnson will be sweating over his skipper's diagnosis, Ireland boss Declan Kidney will have concerns surrounding hooker Rory Best.
The Ulster captain damaged ribs in his side's hard-fought win over Biarritz but the extent of the injury is not yet known.
With Munster No2 Jerry Flannery having suffered yet another injury a fortnight ago, Best is vying with Connacht's Sean Cronin and Flannery's clubmate Damien Varley for a Six Nations start.