Winter weather, a dodgy pitch, a fired-up opponent and Nelsen's experience all tell him to expect a difficult afternoon.
The former New Zealand international isn't about to rest on his laurels after his new-look team's 2-1 win in Seattle last weekend.
"I've been in the game long enough to know now that as soon as you start patting yourself on the back, somebody knees you in the ...," Nelsen said Friday with a laugh, not completing the sentence.
"So we have to get our feet right back on the ground and get ready for this game, because this will be an extremely difficult game."
D.C. United had a bye last week after losing its season opener 3-0 to the visiting Columbus Crew. So they have had two weeks to prepare for Toronto.
Environment Canada called for "a mixture of snow and freezing rain" Friday night, saying "a potent disturbance currently over Minnesota is expected to carve a path across Southern Ontario."
"I might be wearing gloves (Saturday)," said striker Jermain Defoe.
So will many in the stands. The first 10,000 fans will receive BMO mittens.
The England star, who scored twice in his MLS debut, seemed unfazed by the Toronto winter that won't end.
"I played in Russia," he said. "It's cold. But I suppose when you run around, you might get warm so it will be fine."
The weather is supposed to improve as the day wears on, which is good given the 4:30 p.m. local kickoff. The forecast high is for five degrees.
But the pitch at BMO Field, an adventure at the best of the times, could still be an issue.
"Obviously with this winter, the ground staff have had virtually no time to do much on the field," Nelsen said. "So it could be interesting, we're not really sure.
"It could be a bowling green, it could be a bit different."
Nelsen, whose team checked out the pitch earlier in the week, said he expects a soggy surface.
The good news is that Toronto is almost at full health for Saturday's game with the return of Brazilian forward Gilberto and English fullback Bradley Orr.
The only Toronto players missing are midfielder Jeremy Hall and teenage striker Jordan Hamilton, both nursing hamstring injuries.
With a 3-24-7 record, D.C. United was one of only two teams to fare worse than Toronto (6-17-11) last season (Chivas USA went 6-20-8). The team has made wholesale changes since then, bringing in forwards Eddie Johnson and Fabian Espindola, former Montreal midfielder Davey Arnaud and defender Jeff Parke, among others.
The visitors have two Canadians on the roster: former Toronto FC defender Nana Attakora and midfielder Kyle Porter.
Nelsen started his career with D.C. United while midfielder Dwayne De Rosario finished his 2011 MVP and Golden Boot year there. D.C. United elected not to pick up his option at the end of last season, setting the stage for the 35-year-old to re-sign with Toronto.
"They're a playoff team, there's no question about that," Nelsen said of D.C. United.
"They'll be stronger, they'll be wanting to make a point ... they can turn over any team anywhere so we just have to be on our game," he added.
Toronto fans will be eager to see more of Defoe and midfielder Michael Bradley, who pulled the strings in Seattle.
Defoe showed off his clinical finishing against Seattle, with two goals in 24 minutes. He has now scored in his debut for West Ham, Tottenham, Portsmouth, Toronto and England.
"A lethal finisher," said D.C. United coach Ben Olsen.
The two goals would have tied him for fourth on the team for scoring the entire 2013 season. Defoe was named MLS player of the week.
Bradley said the players have had the home opener circled on the calendar for weeks.
"So it's up to us to make sure we step on the field and play in a way that people are excited about, that people are proud of," he said. "It's going to be a good afternoon."
Toronto fielded six new starters in Seattle and two more newcomers could make their MLS debut Saturday in Gilberto and Orr.
Friday's practice at the team's north Toronto training centre drew some two dozen media. You could count reporters at training sessions last season on one hand, and still have a few fingers left over.
There were still some tickets available for the game on Ticketmaster, as of Friday afternoon.
StubHub, a ticket reseller, says sales on its site for the home opener have more than doubled last year's opener. Last week's win saw a 90 per cent increase in page views for Toronto FC and a 150 per cent increase in ticket sales for the home opener, it added.
NOTES — Nelsen said Toronto is close to signing Canadian international winger Issey Nakajima-Farran ... Former Columbus Crew defender-midfielder Danny O'Rourke is training with Toronto. He actually belonged to Toronto briefly in 2006, when he went second in TFC's expansion draft, but he was traded to Columbus soon afterwards. Nelsen said O'Rourke is training with Toronto for a few days "and then we'll go from there."