Some have learned the hard way.
After missing a game against FC Dallas the previous week due to a stomach virus, a healthy Danny Califf found himself on the bench Saturday in Philadelphia. The 33-year-old centre back, who was the Union's first captain and played 65 games for Philadelphia, was looking forward to playing before friends and family in his 212th career MLS game.
But Nelsen stuck with 21-year-old replacement Gale Agbossoumonde, who had hardly put a foot wrong against Dallas in his MLS debut.
Agbossoumonde acquitted himself well again in the 1-1 tie with Philadelphia. Califf did not leave the bench, perhaps in part due to the fact that Nelsen had to swap injured fullback Darel Russell after just seven minutes.
Nelsen said it was a tough decision to keep Califf off the pitch in Philadelphia.
"Really difficult," Nelsen said after Tuesday's practice. "Danny's been such a good player in this league. He's been a great pro. He's been great with the younger players ... It's really difficult but it's a great decision to have, it's a great position to be in."
Goalkeeper Stefan Frei, the team's vice-captain coming into the season, was looking forward to playing again after recovering from a leg and ankle injury that cost him all of last season. A broken nose in the opening pre-season game opened the door to Joe Bendik and the 23-year-old backup has made the starting job his own, most recently with a string of dazzling saves in Philadelphia.
Bendik is up for MLS save of the week for his diving stop of Brazilian Kleberson.
Unheralded 20-year-old midfielder Jonathan Osorio earned a place in the starting 11 in the first pre-season game and used some useful appearances as a substitute to step into the lineup when Terry Dunfield went down.
But what goes up can come down. A healthy Luis Silva has now moved into the starting 11, with Osorio seeing just 17 minutes action in relief of Silva in Philadelphia.
The TFC players know that every time they step on the field, they can make a claim to stay there if they perform.
"That's kind of the identity of this team," said fullback Ryan Richter, who was called on to replace Russell. "We're going to go out there and we're going to fight and no matter who gets thrown in the game, we're going to fight for a win."
Or fight for a job. On a personal level, it makes for a harsh credo.
Imagine you miss a day's work with a stomach virus, only to come back and find that someone else is sitting at your desk?
Califf didn't complain. But on Tuesday, he was breaking a sweat with the reserves against the No. 1 squad.
"That's the brutal reality of professional sports," said Nelsen. "But the way Danny's been training, it's been great.
"You can go two ways when things like this happen. You can get the sulks or you can puff your chest out and try and get better each day. And that's what Danny's doing. We try and challenge all the players here — they're not setbacks, they're not things that are meant to get you down. They're just opportunities for you to get better and try to improve yourself.
"Danny's come in and he's trained very well. He's going to cause more selection issues very soon."
Nelsen's backline options are limited this week, with the Houston Dynamo (4-2-0) visiting Saturday.
Ashtone Morgan is suspended after being sent off Saturday and fellow fullbacks Russell and Richard Eckersley are both injured.
Nelsen used a back four of Richter, Darren O'Dea, Agbossoumonde and 19-year-old Doneil Henry in practice Tuesday, the first outdoor session of the season at the club's north Toronto training grounds.
Nelsen complimented Richter's performance in relief of Russell, calling it "fearless." And he has been talking up Henry's performance in practice for days.
TFC's youngsters are "turning into men," Nelsen noted.
The former New Zealand international will need all his resources in the weeks ahead.
After Saturday's game against Houston at BMO Field, the schedule gets more crowded with midweek Canadian Championship games and weekend league outings the next two weeks for Toronto (1-2-3).
"It's a real test for a squad and we're going to be using the squad," said Nelsen. "So it's an opportunity to stake a claim and if you do well in one of those games, then it's hard for me to not pick you."
Frei, 26, may get his long-awaited chance to play in the Canadian Championship game, although Nelsen made it clear his goal does not change be it league or cup.
"All we care about at this club is winning," he said.
"The players that put on a Toronto shirt and walk out (over) that white line and represent the club are expected certain standards," he added. "And if they drop them or don't adhere to them, then there's no future at the club."
NOTES — Toronto captain O'Dea complained after Saturday's tie in Philadelphia that "being from Toronto, you don't get much" in MLS. The plain-speaking Irish international elected not to talk to the media Tuesday. Nelsen played down the comments, saying calls tend to even out over the season. ... Dunfield has a torn posterior cruciate ligament and meniscus and will be out another four weeks before being re-evaluated by the doctor.