In the 37th minute of a first half controlled by the champions defender Efe Ambrose scored his second goal in two games and the Hoops had chances to put the game to bed.
The Perth side made a better fist of the second half and had the ball in the net twice through Steven MacLean and then Fraser Wright, only to see both efforts ruled out for offside.
However, with eight minutes remaining substitute Nigel Hasselbaink, on for Rowan Vine, curled a fantastic shot past Fraser Forster to stun the Parkhead side.
Lomas said: "It is an old cliche but it was game of two halves and we are not stupid, we know it could have been well and truly over by 30 minutes and we could have been looking at a long evening if it wasn't for Alan, who made four or five great saves, and the back four.
"You have to hold your hands up and say Celtic's passing and movement was a joy to watch.
"But we had a change for the second half when we put Gregory Tade on, went to four in midfield and were then able to compete and I think we were well worth it in the second half."