Chelsea winger Christian Pulisic has picked up a "nasty" thigh injury in training after a hectic Christmas period, manager Frank Lampard said in a news conference on Friday.

U.S. international Pulisic will be out for a few weeks after reporting tiredness following a busy schedule of games over the festive period.

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Pulisic picked up the injury in training prior to the club's 2-0 win over Nottingham Forrest in the FA Cup third round on Sunday.

"It is a damaging miss," Lampard said. "Before Christmas he did report a bit of tiredness so he came out of the team and I was worried about him picking up an injury.

"He was in pain when it happened. He's a good player that provides competition in wide areas and can make things happen. He was in good form for a period.

"He picked up an injury a day before the Forrest game. [An] injury in the abductor, a tendon injury. Nasty injury, he was in paid when it happened."

The 21 year-old scored a hat-trick against Saturday's Premier League opponents Burnley earlier in the season following a lack of first team football when he first arrived at Stamford Bridge last summer. Pulisic has become a regular starter for Chelsea since then, but has failed to play for 90 minutes since playing Everton on Dec. 7.

Pulisic signed for Chelsea from Borussia Dortmund 12 months ago for a fee of £57.6 million, but was immediately loaned back to the Bundesliga side before joining the Blues this summer.

He will not be in contention when Chelsea face Burnley at Stamford Bridge on Saturday, with Lampard's side hoping to bounce back after losing their last two home games.

Lampard said striker Olivier Giroud can leave in the January transfer window if the right deal can be found, but added there was no agreement with any club. Chelsea have scored just three goals in their past five homes games in the league.

"We created 53 chances in our past three league games and scored no goals," he added. "It's going to hurt you in the league.

"We get into the final third a lot but we are not converting chances. In terms of play, we're controlling big parts of games, but we're not taking our chances.

"We have to get things right and we work on that in training. We need to find that killer instinct."

Lampard said the club are still looking to sign a striker in January but that forwards are in short supply at this time of the year.