Allen says Dortmund will not find things easy when they arrive on Merseyside.
Both sides made several changes for their respective games. Liverpool made seven changes for their 4-1 win over Stoke City while Thomas Tuchel's man saw eight changes to their starting XI for the 2-2 draw with Schalke.
The first leg between the two sides at Signal Iduna Park finished in a 1-1 draw, giving the Reds a crucial away goal going into the second leg.
Allen told the official Liverpool website after playing his role in their emphatic win over Stoke: "It's difficult when you've got a game in between an exciting tie like the one we've got, but we wanted to focus on today and make sure we didn't have one eye on that game.
"But we can celebrate today's game and turn our attentions now to what will be a massive game on Thursday.
"It's a stellar tie against a very tough team but we went out there and gave a good account of ourselves and got a good result.
"So I think the onus is on them now to come to Anfield and it certainly won't be easy for them."
The task ahead of Liverpool is still a big one but after a positive result in Germany, followed by an emphatic win in the Premier League with a much-changed team, it will be a confident Anfield getting behind a side with something of a spring in their step as the two sides face off for a place in the last four.
Whichever side progresses will certainly fancy their chances of making it to Basel and lifting the trophy.