Kai Havertz said he expects "a big turnaround" in his fortunes at Chelsea now he is free of injury and illness, and that there can be no excuses if he fails to deliver at Stamford Bridge.
The German midfielder has scored just once in 21 Premier League appearances since arriving from Bayer Leverkusen for $98.20 million at the start of the season.
However, a combination of Covid-19 and an apparent lack of trust by former manager Frank Lampard saw Havertz, 21, used mostly as a substitute before picking up an injury last month.
"The coronavirus is now over [for me] and I feel good," Havertz told Chelsea's website. "I had a little injury in the past few weeks but it's now time to play good.
"It has not been easy for me but there are no excuses.
"I just have to keep on working and hopefully there will be a big turnaround soon."
Chelsea host Atletico Madrid in the second leg of their Champions League last-16 tie on Wednesday. They won the first leg against La Liga side 1-0 in Bucharest.
The Blues are unbeaten under Thomas Tuchel since the German replaced Lampard in January.