The 45-year-old took charge of his first game as Black Cats boss but had to contend with three Swansea goals in eight minutes as his side capitulated in the second half.
|7/4||Sunderland are 7/4 with BetVictor to beat Newcastle United next week
"[We were a] completely different team in the second half," he told Sky Sports News. "I think it’s been happening for a while. It’s a team when it’s in the game [and] is fit enough, we were doing things naturally.
"Then when something happens it goes against the team [and] it changes completely. I learned a lot today. I learn a lot from individual players, from situations from reactions and I’m sure I can put that right.
"In certain aspects we need to improve. We will analyse all of them: the technical, the physical, the tactical. But of course there is a problem that we need to address and the most important is the mental sign.
"Nowadays in football, in any sport, you need to be strong mentally. This is not about one person, it is about everybody at the club – especially the level that they are on the pitch. Maybe there is more work mentally than the rest."
The former Brighton manager also rued the lack of stability in a defence that conceded two deflected own goals from corners but wants to bounce back with a victory as soon as possible, starting with Newcastle next week.
He continued: "We had a couple of half chances in the first half – one from a corner and a couple of breaks. But we are a little bit too [much] in a hurry with the ball so we don’t care enough about the ball.
"On the other hand, as soon as something happens in the box, it’s a deflection or whatever and it’s going in. That’s how it is.
"We’re not going to change here apart from working, making sure we do the same things 90 minutes and winning one football game. It sounds easy. It’s winning a football game so we should go there and make sure we do it as soon as possible."