The Black Cats were excellent through the first half of the game and took a deserved lead through Jack Clarke, but Coventry City's four substitutions changed the game and their late equaliser through Victor Gyokeres was well deserved.
Neil said the step up in intensity at the second tier was clear, and underlined the need for a big and competitive squad.
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"We've still got four, potentially five that we want to bring in before the window ends," Neil said.
"We need to make sure the squad is competitive and not only that, they brought four or five really fresh lads on who know the level, and that can really make a difference.
"I think what you could see today is that it was a step up in intensity. I thought towards the end of the game our lads were dead on their feet, it really is difficult. Normally we score a goal in League One, the opposition gets a bit scared and they sink back into their shape. We control the ball and it's basically a case of when will the next goal come.
"At this level, Coventry came here thinking, 'we're better than Sunderland and we have better players, we're coming to win'.
"The intensity was one of the big learning curves for the lads today in terms of what it's going to take to play at this level.
"They're going to have to play for 96 minutes plus, every single week. There's not going to be a minute off."
On the game itself Neil said: "I thought it was a tough match, first and foremost.
"They’ll be aiming to try to be a top-ten team this season, I’d imagine, having been in the Championship for four or five seasons. "They’ve built year or year, and you can see they’ve now got guys that other teams are willing to pay £8-10m for. We knew it was going to be tough. I thought our start was excellent, we moved the ball really well and scored a really good goal.
"I thought we had really good moments. We knew Coventry were going to have some territory, but I thought we really limited them.
"Let’s be honest, we’d be talking about a great result here if it wasn’t for one moment of quality. We switch off, don’t get down the line quick enough and let him turn, but even then, it’s a great, great goal.
"It’s not as if we’ve made an error and they’ve had a tap in. The lad’s had to turn, dip past two players and bang the ball in the bottom corner. That’s the quality of the Championship.
"They’re a good side, with good players, and they were always going to make it difficult. All in all, for what our players offered up, I was really pleased with our effort and how we went about it.”