Rose Lavelle suffered a knee injury in Manchester City's win over Everton on Sunday, though it is too soon for City to know the extent of the problem.
Lavelle, who won the 2019 Women's World Cup with the United States, was withdrawn at half-time, having suffered the knock towards the end of the first half.
The 25-year-old required on-pitch treatment after winning the ball back with a lunging tackle and was walking on crutches after the interval, though not limping heavily.
Speaking after the game, City head coach Gareth Taylor said: "I haven't been in [the changing room] yet. I've seen obviously that she's on crutches. I saw a clip of her going down, it just looked a little bit awkward on the landing, when her knee was planted on the pitch.
"We just have to see. Hopefully it's not too bad. She's quite a resilient character, Rose, and she doesn't go down for much at all. Fingers crossed she's okay."
It was a premature end to what was just Lavelle's second start in the league for City, whom she joined in the summer. Her quick feet had dazzled during a dominant first-half performance by her team as well.
The creative midfielder arrived in Manchester with an ankle injury, which delayed her debut until October, when she came off the bench against Tottenham Hotspur in a 4-1 win.
After starring in the USWNT's 2-0 win over the Netherlands during the international break, scoring a stunning goal reminiscent of that she scored against the same opposition in last summer's World Cup final, Lavelle appeared to be getting back to somewhere near full fitness.
However, this is yet another setback for her and for City, who have a very busy month of fixtures coming their way as the Women's Champions League knockout rounds get underway.
City will travel to Sweden this week to face Goteborg in the first leg of their last-32 clash, before a huge game with Arsenal, their fellow WSL title challengers, on Sunday.
The following week will see Goteborg travel to England for the second leg of that European tie before a game against Birmingham City rounds off their year, with the Christmas break following.
There were already signs of Taylor preparing for that run of fixtures on Sunday against Everton, with Janine Beckie making her first league start for two months, Gemma Bonner starting after not featuring in the last two games and Jill Scott making her first league start of the season.
Both Beckie and Bonner were on the scoresheet too, along with striker Ellen White, as City won 3-0 to move up to fourth in the WSL table.