Not only will Neil Lennon's men be eager for revenge against St Johnstone, who won 2-1 in the SPL last month, but they will also be determined to respond to Saturday's home loss to Kilmarnock.
"We're massive underdogs but we go there in good heart," Lomas said. "We've seen what Kilmarnock were able to achieve. Neil's been saying they want a response, so it's probably the worst time to play Celtic."
Prior to Saturday's loss to Killie, Celtic's only defeats this term were against St Johnstone and at Barcelona last week.
Lomas downplayed the significance of his side's 2-1 win on September 15, a result which sparked a seven-match unbeaten run.
"It's a totally different game," Lomas said. "It's about who performs on the night. We're under no illusions it will be very, very difficult.
"We're really looking forward to it. The boys always enjoy playing at Celtic. We're going to go there, give it a right go, given the size of the task."
Lomas, who came through the youth system at Manchester City with Lennon, was among those to wish Celtic well, sending his friend a text message.
Lomas added: "I just wished him good luck. In Barcelona I thought they played really well and were unfortunate not to get what would have been a great result."