The 28-year-old has recently returned to the United side after a lengthy finger injury and could be in for a busy evening at the Emirates as the Gunners embark on their quest to earn cup glory for the first time since 2005.
Whilst red-hot striker Robin van Persie looks set to be rested in the tie, which is a repeat of last season's third-round encounter Leeds were minutes away from winning before a late Arsenal penalty earned a replay and a subsequent 3-1 win, Lonergan knows whoever he comes up against will be of the highest class.
That is likely to include the Gunners' all-time leading scoring Thierry Henry, whose loan move from the New York Red Bulls was completed yesterday and he is in line to make his second debut.
Lonergan expects the spotlight to be firmly on the 34-year-old, but is wary of all of Arsenal's forward players.
"The goals that go in are fantastic in the Premier League and you feel for the keepers because the level of finishing is unbelievable," Lonergan said.
"If they get a one on one or a chance, they don't think about it, the next thing you know there is a shot coming.
"That's the difference between our level and the Premier League.
"Every league you go in the difference is always the strikers and we're playing a Champions League team so the finishing is clinical.
"Henry is probably one of the best players to play in the Premier League, probably the world.
"He is fantastic and it will be a pleasure to share the pitch with him hopefully.
"He won't have a point to prove. If he plays there will be a lot of attention on him, but there will be 10 others who are equally capable of hurting us."
This is not the first time the npower Championship club have come up against Premier League opposition this season as they faced bitter rivals Manchester United in the Carling Cup in September and were on the wrong end of a 3-0 hiding at Elland Road.
Lonergan, who played in that game, knows Leeds will have to perform better than they did that night if they are to get anything at the Emirates.
"We want to give a better account of ourselves than we did against Manchester United because we didn't get near them," he added.
"I don't know what happened that day, we didn't do the things we had been doing because we were on a good run at the time.
"A big deal has been made about our league position. This gives us a different kind of pressure. We can go and enjoy it a bit more."