Frank Lampard admitted Timo Werner was well off the pace before being replaced at half-time against Arsenal.
Chelsea lost 3-1 at the Emirates Stadium, where boss Lampard withdrew Werner and midfielder Kovacic at half-time.
German striker Werner has not scored in 10 matches and continues to struggle to adapt to the Premier League.
“Certainly the goals not going in is something an attacking player will get judged on,” said Lampard.
“With the subs, I could have made more subs. I felt I had to make subs in those two areas to inject energy.
“Timo wasn’t giving us enough, with or without the ball.
“With some of it we have to give him more time. But also we have to get there quickly.”
Chelsea were 2-0 down after a woeful first-half performance against their London rivals.
And Lampard admitted: “There was definitely a lethargic approach to the first half.
“If you come and play slow, on and off the ball, you’re going to have a big problem trying to get a result.
“We had to attack in the right way and we didn’t.”