Christian Pulisic is a doubt for the USMNT this month after being ruled out of Chelsea's final Premier League game before the international break due to an ongoing hamstring problem.
The 22-year-old has started just three matches this season, making only five appearances in total as he battles to regain full fitness following an injury originally sustained in August's FA Cup final defeat to Arsenal.
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Pulisic had made slow progress prior to breaking down in the warm-up before the win at Burnley last weekend, forcing his withdrawal from the starting line-up and subsequent absence from Wednesday's Champions League victory over Rennes.
The setback did not prevent USMNT head coach Gregg Berhalter naming Pulisic in a 24-man squad for upcoming matches against Wales and Panama. However, Lampard said Pulisic was no closer to making a full recovery after admitting in midweek a scan had revealed a "very, very minor injury" which has prevented him from training.
"He's not fit to play [against Sheffield United], he is still suffering with his hamstring," Lampard said. "He is not [training] at the moment, no.
"To be clear, he trained at a low level in midweek when I made those quotes and he wasn't comfortable so we have to keep looking for the right solution."
Lampard reiterated his claim from earlier this season during the week that Chelsea are searching for the best way to handle Pulisic for fear of injuries hampering his progress after a promising first season in England following a £58 million move from Borussia Dortmund.
"I don't think that's a one-size-fits-all answer, because it's different for everybody," Lampard said on Tuesday.
"From last season we were looking at ways of managing Christian and looking at ways of being proactive in not hopefully getting so many injuries, I know that's something he's experienced previously in his career at Dortmund as well.
"So we're all working in the same direction on that one, to try to get him to be as fit as regularly as possible, because we all know his talent."
The U.S. will definitely be without forward Josh Sargent after club side Werder Bremen declined to release him, citing COVID-19-related quarantines in Germany.
The four other Bundesliga clubs with USMNT players -- Bayern Munich, Borussia Dortmund, Wolfsburg and Leipzig -- will allow players to travel and are set to release Chris Richards, Gio Reyna, John Brooks and Tyler Adams.
However, sources have told ESPN that Adams could remain in Germany in a bid to manage his workload as he has struggled with minor injuries during the early stages of the season.
He has appeared in six games but has only started one game since early October. After not featuring in Leipzig's previous three games he came on for 14 minutes during the 2-1 win over Paris Saint-Germain in the Champions League on Wednesday.
ESPN's Stephan Uersfeld contributed to this report.