Undaunted by the tough fixtures coming up, he says: "If we win on Wednesday against Aston Villa we can relaunch ourselves - the objective is to finish in the top 10.
"That game and the trip to Manchester are important games for us and the supporters."
Sebastien played the second game of his Hammers career in the famous FA Cup victory at Old Trafford - a performance that really earned him recognition back in France.
"When I arrived here I played against them and they had a side 10 times better then," he says.
"We need to go there with a desire to win and, above all, being convinced we can't lose.
"We have the means to win and Manchester United is not a team we fear.
"You have to show yourselves in these games and we have nothing to lose.
"West Ham can win anywhere; we should've won against Sunderland and we can win at Old Trafford."
He feels the criticism of fellow countryman Laurent Blanc is unjustified and says: "Blanc is criticised because they are conceding goals but he showed in the World Cup he is one of the best defenders in the world.
"I haven't seen Manchester United this year but for France he was one of the best players in the world; maybe the players around him are not at his level.
"He is still and will always remain a great player."
As for the Sunderland defeat, he concludes: "We deserved to win easily but we lost out on one error in the game - that's football.
"We have shown we are not in our place; we are twice as good as Sunderland, but despite playing well we didn't pick up any points, and we must do that on Wednesday.
"In the space of three minutes we have had chances to make it 3-0 and then the match is finished - they get two chances and score one - and that is the reality of it.
"We were very, very, disappointed and are sorry for the supporters who came in their numbers."