Easter, who played a major role in his country's successful World Cup campaign, was expected to be named in Brian Ashton's squad for the tournament when it is announced on January 9 - but the England head coach now faces a nervous wait until the extent of the forward's injury is known.
Quins director of rugby Dean Richards said: "Nick has twisted his knee.
"We'll see how he is and we'll make sure he has a scan in the next couple of days."
Victory ensured the London club remain in the hunt to play in next season's Heineken Cup, but Easter's injury was a massive blow in a game Worcester targeted as an ideal opportunity to claim their first league win of the season in their ninth start..
Richards said: "I don't think it's easy to get a win at Sixways. You only have to look at the introduction of the three southern hemisphere boys to appreciate that on paper they don't have a bad side at all.
"It's fair to say that I'd have been happy beforehand if I'd been told we'd win 10-7.
"It got to half-time and we thought, 'great, we'll have the wind going down the right hand side and we can put the ball there', and as we came out it started to rain.
"That put the kibosh on a lot of things. It was a real shame. The weather closed us down quite a lot in the second half.
"The scrum-half found it difficult to put it on the sweet spot every time for the number 10 and that was whether it was their nine or ours, and that was because of the wind.
"The conditions were difficult, especially in the second half."
Quins scored two first-half tries through Chris Robshaw and hooker Gary Botha while replacement fly-half Loki Crichton touched down two minutes from time for the try that sealed a losing bonus point for the hosts.
Warriors director of rugby Mike Ruddock said: "Harlequins got the first one in and that was important. We were always chasing the game after that.
"Harlequins' half-backs did particularly well to control the game, get us chasing the ball and we were chasing the game from there on in.
"We've always given our boys a licence to play a quick-tempo game, but we were frustrated and couldn't get any quick ball.
"It was a difficult day all round for half-backs in those conditions. It was blustery and passing was difficult.
"Harlequins did their homework and were always in front. We believe we are potentially a very good team and we need to get some wins together to move up the table.
"It's important we scrap for every point and we did."