Saints are unbeaten in three games against the Hoops in the Clydesdale Bank Premier League this season and beat them at McDiarmid Park in September.
But Lomas said: "I don't think that counts for anything. It will be a different game. We did well in those games but this is a different match.
"We have to be on top of our game, we have to be resolute and we have to play with high energy, and if we get chances then we have to take them. But I have been telling the boys all week that you don't get away with being average against Celtic."
Celtic will collect the SPL trophy after the match and there is sure to be a higher-than-average crowd to celebrate the occasion.
But St Johnstone are focused on securing success of their own as they seek a second consecutive season in Europe.
Victory will put them ahead of Inverness before Caley Thistle host Dundee United, but defeat would allow Terry Butcher's side the opportunity to wrap up a Europa League spot.
Lomas said: "It's a massive game. They are collecting the trophy and there will be a partisan atmosphere but we have our own agenda to get a result that keeps the European race alive going into the final game. It's a great challenge for the boys to go there - can we ruin their party?"
Lomas, whose side are unbeaten in six games and secured wins over Inverness and Dundee United in their previous two outings, added: "We know it's going to be a tough match but we have been on a decent run of form.
"We are in there fighting and we are trying to take it to the last game of the season."