According to Le 10 Sport (h/t the Mirror), Chelsea want in the region of £27 million for the Brazilian, with that money to be spent on his compatriot.
Willian has made 198 appearances for the Blues since his arrival in 2013, producing 32 goals and 30 assists in that time.
The 29-year-old, who has won 51 caps for Brazil, drew Chelsea fans' ire for his performance against Manchester City earlier in the season, per Bleacher Report's Garry Hayes:
However, he is typically a dependable performer whose work rate helps the side defend from the front as he presses to regain possession in the opponent's half.
He's a real threat from set pieces, too, per OptaJoe:
While the Blues should perhaps not be in a rush to cast Willian aside, Lucas could make for a useful player at Stamford Bridge.
With the likes of Neymar and Kylian Mbappe to compete with, not to mention Julian Draxler and Angel Di Maria, Lucas has played just 71 minutes of football this season split between five cameo appearances, though he has still managed to contribute a goal and an assist in that time.
The 25-year-old had appeared to lose his place even before Neymar and Mbappe were in the team, per ESPN FC's Jonathan Johnson:
That's difficult to explain after he racked up 19 goals and 11 assists last season, which was his best campaign in a PSG shirt and one that suggested his problems with inconsistency were becoming a thing of the past.
It appears he has little future at the Parc des Princes, and his sale can contribute to PSG's efforts to keep in line with financial fair play regulations, so he should be attainable in January.
As things stand, he would be eligible to play in the UEFA Champions League as he's yet to feature in the competition this season.
He can provide a greater attacking threat than Willian—from open play, at least—so he could be a good option for Chelsea in the final third.