Giroud, 33, has been linked with a move to Premier League rivals Tottenham as well as a switch to Italy with Inter Milan and Lazio.
However, the France international is now set to remain at Stamford Bridge after the club were unable to agree a deal to bring in Napoli striker Mertens to replace him.
When asked if Mertens would join, Lampard, who appeared agitated, said: "No," and added: "I wouldn't say frustrated. I wanted to bring players in, but I know how football has been.
"The window is 95% shut for us. The answer is probably no for the players you would ask about."
When pushed on Giroud's situation, Lampard said: "He has been incredible as a player and a man during this time. Will he leave? No. He trained today. No ins and no outs."
There was better news about Tammy Abraham's fitness with Lampard confirming the striker trained on Friday and felt OK, something he described as positive.
Abraham will undergo a fitness test on Saturday morning ahead of the Premier League clash with Leicester.