The summer signing, who joined Newcastle from Leicester City, was substituted in the 12th minute at Bramall Lane after suffering a foot injury.
"I think it is quite a substantial injury. We’ve had it scanned, we’re waiting for a specialist opinion on what to do next, whether there’s surgery involved or not.”
Barnes has scored one goal and assisted another in six Premier League appearances this season.
The 25-year-old's absence could benefit Anthony Gordon, who has been sharing pitch time with Barnes on the left wing for Newcastle. The former Everton midfielder produced a goal and an assist off the bench at Sheff Utd, and Howe has been pleased with how he has begun the campaign.
“He’s had an outstanding start to the season. What’s pleased me the most is his fitness levels, his athleticism is really coming to the fore, but he’s adding goals and [end] product.”
Gordon has enjoyed a strong start to the Premier League season for the Magpies, completing 10 dribbles, more than any team-mate, while only Callum Wilson, with 15, has bettered his total of 12 shots.
He's also impressed with his creativity, ranking joint-second among Newcastle players for chances created from open play with eight, one behind Kieran Trippier.