Despite going ahead after 22 minutes, Saints were pegged back as the holders responded strongly with two goals inside the last ten minutes of the first half.
And the Blues really turned on the style after the break to add a further three strikes to their tally and dump their hosts out of the FA Cup at the third-round stage.
"I think they proved why they won the competition last year, why they are where they are in the league and why they are European champions," Ward-Prowse told Saints Player
after the game.
"We contained them quite well in the first half and Jay [Rodriguez] took a great goal from Jason Puncheon's pass so it looked good, but the two goals before half time killed us.
"Our main priority is the league, so we'll draw a line under it and get going again on Monday to plan ahead for the rest of the season."
Having made his first start in over two months, Ward-Prowse said he was glad to have had the opportunity to test himself against the likes of Eden Hazard, Juan Mata and Frank Lampard.
"It was a massive confidence-boost to be playing again," said the 18 year-old.
"We were playing against international players and World Cup winners, so that was a great challenge and something I embrace," said the 18 year-old.
"I've learned a lot of lessons, both individually and collectively as a team. It will make me a better player for sure.
"This time last year, I'd have never believed that I'd do this. There have been a lot of strides in a short space of time and I am learning all the time – and the main thing is that I'm enjoying it as well. As long as that continues, then things are going well.
"I don't know if I've taken my chance or not – that will depend on the manager's decision up at Villa," added Ward-Prowse ahead of next weekend's trip to Aston Villa.
Saints are currently a point behind the 16th-placed Villains with a game in hand, meaning there is plenty riding on Saturday's visit to the Midlands.
"They are around us in the league, so it's important we get the win," said James. "We'll recover now and get ready for that.
"It will be good to have some time on the training ground to prepare for our next game."