"Sunderland are the same value as us, shall we say, and we went there just like we did at Manchester United and won in the FA Cup - so we can do it again," he says.
"We are capable of going anywhere and winning - we just need a little boost and the injured players coming back."
With Michael Carrick and Shaka Hislop losing their ever present status after missing the defeat against Spurs on Saturday, Sebastien is only one of a trio of players (along with Christian Dailly and Trevor Sinclair) to have played in every game now.
"Everyone was disappointed," he admits, "it was a derby game that was there to be won, or especially not lose.
"There were good players missing but that was not the problem; Tottenham were good despite our quality.
"There were not many chances and it was the first time that Paul Kitson and Jermain Defoe had played together in the first team so perhaps the game came too quickly for them, but I have no worries about them and am still encouraged for the future.
"But it was a match we should have won, just like the Charlton game."
Sebastien thinks it will only be a matter of time before the Hammers get into the top half of the table, and indeed a win at the weekend would see Glenn Roeder's team leapfrog Sunderland.
"We should be in the top 10 now and we have some important matches coming up," he insists.
"But with this team I am very confident - despite the disappointment of losing to Tottenham."