The hosts were trailing 3-1 against a Doncaster side that had been reduced to 10 men when it was ruled that the water-soaked surface was unfit for football.
As the rain lashed down, the match was suspended after just 26 minutes. Doncaster players helped the home groundsman attempt to clear the surface water and when play resumed half an hour later Wales striker Simon Church fired in a reply for Charlton in the 36th minute.
Yet during the half-time break it was decided the match should not go any further.
Some fans were still taking their seats when Keegan headed the visitors into the lead after just two minutes.
Winger Theo Robinson's cross was met by the midfielder who charged into the box as Charlton's back four were caught napping.
Flash showers saw patches of standing water develop on the pitch and home goalkeeper Ben Hamer was almost caught out as the ball stopped still in the box during the opening exchanges.
Soon after the opener he was again beaten as Brown doubled Doncaster's lead in the 16th minute from another Robinson cross.
In the visitors' goal Ross Turnbull was having a quieter game, but showed his class with reaction saves from Bradley Pritchard and Yann Kermorgant in quick succession.
Hamer, however, was having no such luck.
Paul Dickov's side were 3-0 up after Michael Morrison's clearance cannoned off Brown's face and sailed into Hamer's net.
Eight Charlton groundsmen with pitchforks were joined by Rovers players James Husband and Robinson as they tried to clear the pitch of standing water when referee Lee Collins took the teams off.
With the action back under way, Church pulled one back for the Addicks when he thumped home a loose ball in the box.
The Wales striker's goal in the 36th minute was his second in as many Championship games.
Charlton's chances of staging a comeback appeared to be given a further boost when Keegan, already booked for dissent, pulled back Kermorgant and picked up his second yellow in the 43rd minute, but the teams did not return for the second half.
- Sports & Recreation