The premier league champions sent scouts to see the Jamaican impress against Stuttgart last Friday.
The 20-year-old set up both goals in the 2-0 win, showing his skill and pace in the process.
Now Chelsea have made their opening gambit but Leverkusen will fight to keep the talented youngster.
Chelsea have been aware of Bailey’s ability since he moved to Europe from Jamaica as a 15-year-old
They followed his progress as he played in Belgium and have seen him mature further during his short spell in the Bundesliga.
Bailey is seen as a potential replacement for Eden Hazard, who remains a long-term target for Real Madrid
Leverkusen do not want to sell the Jamaican and have rebuffed Chelsea’s initial approach ahead of the January window.
Manchester united, Liverpool and Barcelona are all monitoring Bailey. Chelsea also have interest in Monaco’s coveted midfielder Thomas Lemar. The Frenchman was the subject of substantial offers from Liverpool and Arsenal last summer and both clubs are expected to return with bids in January