Rangers suffer second Old Firm defeat in twelve days
Smith saw his side slump to their second defeat in 12 days to their Glasgow neighbours as headers from David Weir and Daniel Cousin were cancelled out by a Scott McDonald brace and a Barry Robson penalty.
"We know it was going to be a big challenge for but it is still in our hands.
"We know if we keep winning then we have a four-point lead so it is our job to handle the games as they come along."
Smith was also angered that Steven Whittaker was sent off for a second yellow card in stoppage time.
The Rangers boss claims that Celtic striker Jan Vennegoor of Hesselink should have been shown a straight red instead of a yellow for his thigh-high challenge on substitute Amdy Faye earlier in the contest.
Smith added: "There is always going to be a physical aspect to the game but I am a little bit aggrieved about Steven Whittaker being sent off.
"His first foul was a booking but the second one was just a tired challenge at the end of the game.
"Vennegoor of Hesselink's challenge warranted more than a yellow card.
"So when you consider the context of the game, we had a player sent off in innocuous circumstances.
"But I think the referee had a good game with the exception of that decision.
"In the end I was a little bit disappointed not to take a point from the match. I thought we were value for at least a point."