'Gers won 2-0 at St Johnstone to climb back to the summit of the league, holding a slender one-point lead over the Hoops who have a game in hand.
"Celtic have to answer the question now," he said.
"It's the first time they haven't been top of the league for a while. It'll be like this from now until the end of the season, with one of us playing on a Saturday then the other on a Sunday.
"It does put added pressure on Celtic against Hibs because we've gone top for the first time since December.
"We know the feeling, when it's been us who have been under pressure playing after them.
"You have to stand up and put on a performance. We've done it against St Johnstone and it's over to Celtic now.
"It's down to the nitty-gritty and you have to win every game. It's not about how you play, it's about results.
"There were questions asked of us after losing to Dundee United on Saturday and failing to go top. The players definitely felt it as much as the fans - we know we let ourselves down.
"Over the last three years it's been unlike us. When we've had the chance to go on and capitalise we've normally done it, so it was a big disappointment.
"We must concentrate on winning the rest of our games."