Lyon suffered its second successive league defeat as it went down 29-12 on Sunday (AEDT), meaning Bordeaux will be top should it overcomes La Rochelle on Monday (AEDT).
Depleted to 14 men twice in the first half, Lyon was 12-3 down by the interval, with Castres, which had two men sent to the sin-bin after the break, scoring tries from Geoffrey Palis and Thomas Combezou before running in another two in the second half, Rory Kockott and Jody Jenneker touching down, to wrap up the points.
Montpellier failed to close the gap on the top two, as it was edged out 29-25 by Racing 92.
It was a frustrating day for debutant Pollard, who played a pivotal role in South Africa's Rugby World Cup triumph but has been sidelined with a facial injury, as the five-eighth missed two penalties and a conversion.
Toulouse took advantage of Montpellier's defeat by beating struggling Agen 13-8 to move into third place with what was a third straight win.
Elsewhere, Brive and Bayonne shared the points in a 6-6 draw in the day's other fixture.