Former England Under-21 international Lallana made his presence felt within 90 seconds of his appearance, stooping low to finish off a move he began.
"He is a class act and scored a great goal when the game was evenly poised. It was a great impact from a very talented player," said Southampton manager Adkins.
Until Lallana's winner the game had been heading for extra-time after Adam Barton had scrappily equalised for Preston in the 52nd minute to cancel out a Jos Hooiveld headed opener in the first half.
It was the centre-half's first goal in English football, having recently signed on-loan from Scottish giants Celtic.
In truth, however, Adkins' team were never severely tested by their League One counterparts and the Saints boss was pleased with the impact of some of his younger players.
Adkins said: "It was a good game for our young players. Ben (Reeves), in particular, played very well on debut. I was really pleased with his contribution. He looked very assured on the ball.
"It's a valuable victory. We were able to start 10 different players and utilise our squad depth.
"We've got a tough week coming up with Burnley, Cardiff and Watford but there is a good unity in the squad and a growing competition for starting places, which is only a good thing to have."
Like the hosts, Preston made 10 changes from their weekend victory over Brentford but Phil Brown was left bemused at his team's performance.
"I was really bamboozled at times at the way we played. We just didn't attack enough in truth," said Brown.
"I thought we defended really well and though they scored two goals I didn't think they created many other clear-cut chances. The frustrating thing for me was that we would win the ball and immediately give it back to them.
"Football is all about application and attitude and we were just simply disappointing."
Brown also admitted that his attention had already turned to Saturday's League One encounter with Tranmere at Deepdale and hoped that the experience at St Mary's would prove valuable to his side.
He added: "We may not have performed like I know we can tonight but playing at the St Mary's stadium will be a great experience for the younger lads. We haven't played on a better surface all year.
"There is no negative response to me for this defeat because we always knew Tranmere is the bigger game and ultimately we will be judged on our league results."