Was he thinking of the perfectly good goal he scored early on that was disallowed for offside?
"In football you get those - next time I might get one where I am offside - the main thing is we won 3-0," he says.
Was it, perhaps, the fact he couldn't make amends later in the game, when, clean through, he was dispossessed in the box?
"I thought I was in the clear but the defender did well," he shrugs.
So what was it that was irking him as he trudged off?
Incredibly, he was simply analysing his own contribution, picking the moments - and this reporter is struggling to think of many - when he could have done even better.
"That's just me, I'm always searching for the perfect game," he smiles, "and I'm often like that even after a big win. It's just that I want to improve on everything I do.
"Some said it was a good performance but I still say I could have done better - but I'm fine now and I'll be looking forward to the next game on Wednesday night.
"I want to score more goals and improve other aspects of my game, but you aren't going to be the perfect footballer at 20.
"There may be a few things I am good at but there lots of things I'm not good at, and I am willing to improve on them.
"Just the fact that we won, and won convincingly was great. Seventh place is good, Glenn has done a great job, and I think the crowd are really taking to him now.
"You can see the organisation in the team, and I think the first half performance was excellent, though it went a bit in the second half. We carved them open and it was just great to play in a game like that."
As the hype builds on the World Cup coming up, and the selection dilemmas faced by Sven-Goran Eriksson, one or two stories at the weekend somewhat ridiculously tried to paint a picture of the club being just as happy if Joe did not go to Japan and South Korea.
Glenn Roeder had merely said that at 20, time was on his side, and that there was the little matter of three more World Cups being well within his reach!
Some observers are now pointing out that it could be a straight battle between Joe and Trevor Sinclair for one of the 'spare' places but Joe says simply: "I'm not really commenting on anything like that because I am just trying to put in my performances - as every other English player in the country is."