He may have turned 40 but the former Scottish international is still playing regularly for Doncaster Rovers and shows no sign of slowing down.
The secret of his longevity? There are probably a variety. One thing, however, is plain to see. Former Leeds manager Dennis Wise should not have written him off.
Sullivan does not dwell on the reasons for his departure from Elland Road but has spent more seasons in the Championship than Leeds over the last three years.
He will always remain grateful to Kevin Blackwell for the opportunity to join Leeds and enjoyed his time playing for the club's supporters.
But 'Donny' is where his heart is now and Sullivan – who moved away from London for the first time in 2004 – has played a major role in the more recent chapters of their amazing footballing fairytale.
A new deal this summer extended his stay for another year – but where or when his career ends remains to be seen.
What a coincidence it would be if a player who started his career with Wimbledon in the top-flight in the Eighties got a final crack at the Premier League with another set of underdogs.
"People keep asking me when I am going to retire but I don't look at it and think 'right, this is going to be my last season.' I just think of it as the start of a new season, three games in, and I am not thinking about quitting.
"I feel fit and healthy and, touch wood, injury-free. You will have to ask the manager how long I can go on for because it all depends on how long he keeps picking me."
Sullivan was one of Sean O'Driscoll's first signings as Rovers manager and has rarely missed a game during the subsequent four years.
This month, he clocked up his 500th league start and next week, against Watford, could make his 600th professional appearance.
With 28 caps for his country and spells at both Tottenham Hotspur and Chelsea under his belt, he has tasted life at the top table. Now, even if Rovers are unable to mount a promotion push, he plans to squeeze every last drop of enjoyment out of his tank.
"Maybe I'm just putting games on the end of my career rather than at the beginning," he says. "I didn't start playing regularly until I was 25 because I couldn't get in the Wimbledon side and I broke my leg which put me out for nine months.
"I would love to say I don't have 'any' problems but I don't wake up feeling 'oh no, I have got to go to work again'. It's still enjoyment and I don't train any less than I used to.
"This move started me off again. It was important to be getting games. Coming to a different environment with different players and a different level of football teaches you a lot.
"All through my career I have been lucky to work with some good goalkeeping coaches and Lee Butler has been brilliant. I am lucky to have 'Butts' because he is a lively character who keeps me going.
"I also think I have been lucky because I have had no joint injuries – and they are the ones that take a toll on your body. I feel fine after games but I do have an ice bath every day after training because I have learned that makes a big difference. It helps my recovery and it gets rid of all the lactic acid.
"Football is different now from when I started. Gone are the pre-seasons where you didn't see a football for two-and-a-half weeks and more managers now understand that our fitness is a lot different to outfield players so we have to do things differently."
Sullivan appears to view 'age' as no more than a number and resents it when people blame that for the occasional mistake.
"Some people were saying I was over the hill 10 years ago!" he recalls.
"It makes me laugh because no-one worries about your age or how many games you have played when things are going well. They say you are using your 'experience'. But as soon as you make mistake, it is because you are 'too old' – even if it is a mistake I also made 15 years ago! That's the way you are portrayed. Age is an easy excuse but that's the way life is."
Rovers host Hull City this afternoon and will need to bounce back from a 4-0 defeat at Cardiff City – their heaviest defeat since returning to the Championship two summers ago.
"Sometimes you just have to look forward and get a result," he said. "We have not brushed it under the carpet but the manager has looked at it in a very productive way and we don't want it to happen again.
"We have had a couple of years now in the Championship and competed well against the top sides. We just have to get the consistency now," he said.
"We have a good manager and the club is very ambitious, it is not a club that just wants to see out its time in the Championship."
Comparisons between 'Donny' and the 'Dons' of Wimbledon are there for all to see – the biggest difference, however, is that one made it all the way from non-league football to the top flight.
"If we can even achieve half the things that Wimbledon did, it will be some achievement," said Sullivan. "Anyone can get up there in the Championship, its just the consistency you need in getting results."
Doncaster v Hull City
Team news: Rovers will assess the fitness of striker Billy Sharp, who is a doubt after picking up a knock in training. Winger John Oster is set to return following knee surgery while defender Shelton Martis may play after recovering from a hamstring injury. Ian Ashbee returns to the Hull side after being rested for the midweek Carling Cup defeat to Brentford. Kamil Zayatte is also available again after recovering from a dead leg, but Caleb Folan (dead leg) and Craig Fagan (groin) are both out. Paul McShane is a major doubt so Nolberto Solano could step in at right-back.
Last six games: Doncaster Rovers WDWLDL, Hull City DDWLDL.
Last time: Doncaster Rovers 1 Hull City 0; January 22, 2005, Championship.
Referee: L Mason (Lancashire).
SkyBet odds: Doncaster Rovers to win 5-4, Hull City to win 11-5, Draw 23-10.