Chelsea imploded in an all-too-familiar manner despite being in the ascendancy during the second half at Emirates Stadium.
A second goal conceded from a corner, scored by Timber in the 66th minute, was followed by a cynical foul from Pedro Neto that earned him a second yellow card and, ultimately, his dismissal.
It was Chelsea's seventh Premier League red card of the season, leaving them two short of the division's record with 10 games still to play. The sending off swung momentum back in Arsenal's favour, and Chelsea were unable to recover.
It was also their ninth red card in all competitions. Before kick-off, head coach Liam Rosenior had suggest discipline was improving under his watch, but he has now seen players sent off in consecutive matches.
Pedro Neto will miss the midweek trip to Aston Villa, creating more headaches for Rosenior: Cole Palmer is a doubt, unable to play three matches a week because of ongoing fitness issues, and fellow attackers Estevao Willian and Jamie Gittens are out injured.
His dismissal came just a week after Wesley Fofana's red card in the 1-1 draw against Burnley. Fofana's suspension allowed Sarr to make his first league start since returning to Chelsea in Sunday's match at Arsenal.
As with last weekend's collapse, when Zian Flemming headed in an added‑time equaliser from a corner, Chelsea "set fire" to points once again, to borrow Rosenior's phrase.
They again demonstrated the contrast between their attacking and defensive set‑piece output. James' deliveries created an own goal, two penalty shouts and a clear chance, but the same vulnerabilities persist at the other end.
And the same old questions will be asked of this Chelsea team - the youngest in the Premier League - who have not beaten Arsenal since the days of Thomas Tuchel's management and Roman Abramovich's ownership.
In the short term, Chelsea have lost further ground on rivals Manchester United and Liverpool, both of whom won this weekend. They will need to improve if they are to realise their ambition of returning to the Champions League for next season.