The 25-year-old playmaker looks a different prospect under the Liverpool manager.
Indeed, not only was the Welshman the penalty shootout hero that booked Liverpool's place in next month's Capital One Cup final, he has been a shining light in midfield whenever he's been given the chance.
Before Klopp's arrival, Allen was the recipient of plenty of justified criticism.
But, now, he looks to have two things plenty of his team-mates lack: energy and desire.
Perhaps Allen's faults were more down to Brendan Rodgers. Not only did the former Liverpool boss overhype him by comparing him to the world's best passers, he then limited him by instructing him to play simple, sideways passes.
That's what it looks like with hindsight - as we aren't seeing the 25-year-old do that anymore.
It's a shame Klopp didn't start Allen against Stoke City on Tuesday night. The Reds may well have won that game in normal time had he done so. When he came on, the Welshman did one thing no one else managed throughout the full 120 minutes - run forward with the ball and pick out the right pass, letting his team-mates advance forward and also push the Stoke defence back.
Under Klopp, Allen looks a totally different player. Can he now go on to be an actual success at Liverpool?