The Brazilian international has been at Stamford Bridge since 2013 after famously pulling out of a move to Tottenham, making 339 appearances across all competitions and winning plenty of silverware including two Premier League titles.
However, Willian's future with Chelsea has been subject to doubt for some time now, with the player rejecting the offer of a new two-year contract before the Covid-19 shutdown and then only agreeing a short-term extension covering the end of the 2019/20 campaign.
The Blues still want to keep the 31-year-old for a further two seasons, but there is considerable interest from elsewhere, with agent Kia Joorabchian having confirmed multiple approaches for his client from the Premier League, Europe and the United States.
Arsenal - who beat Chelsea 2-1 in the FA Cup Final at Wembley on Saturday, with Willian again absent due to injury despite initially being expected to return - have also made a contract offer, with multiple reports of talks between the two parties circulating throughout Monday.
Willian's insistence on the security of a three-year contract is believed to be the main sticking point with regards to negotiations with Chelsea, whose season will officially conclude this weekend if they are unable to overturn a 3-0 deficit against Bundesliga winners Bayern Munich in their Champions League last-16 second-leg tie at the Allianz Arena on Saturday.
Manager Lampard has repeatedly voiced his desire for Willian to stay put and praised his consistent form in the face of such fervent speculation.
At this stage he appears to remain none the wiser as to the player's next move, but says he will respect the final decision - whatever it may be.
"I've got a great relationship with Willian. I actually don't know what that decision is," Lampard said.
"If that happens over the next few days, as he said, that will be good to me in either way.
"Of course I'm very happy with Willian. He's been brilliant with me this season with input and work ethic within the squad.
"But it is his choice and I respect his choice.
"He's been a great servant for Chelsea if he decides to move on and if he does decide to move on, Chelsea obviously moves on as well. We have to look forward ourselves."