There is a loophole, however.
A Major League Soccer spokeswoman said Monday that clubs can designate one player on an MLS contract as a yearlong USL loanee, with that player off the MLS books.
That brings Toronto back down to 28.
The MLS club did not immediately identify which player had been designated as the yearlong loanee.
Toronto currently has four players on its roster who are on loan to USL feeder club Toronto FC 2: goalkeepers Alex Bono and Quillan Roberts and midfielders Manny Aparicio and Chris Mannella.
Toronto FC 2's 12 other players are signed to USL deals.
Bono was signed to a Generation Adidas contract before the 2015 MLS SuperDraft, meaning his contract does not count against Toronto's salary cap.
Prior to the most recent collecting bargaining agreement, MLS teams were allowed 30-man rosters. That has been reduced to 28 under the current labour deal.
Simonin, who had been on a USL contract with Toronto FC 2, was upgraded to an MLS deal by necessity.
Heading into Sunday's game against Real Salt Lake, defenders Steven Caldwell, Eriq Zavaleta and Mark Bloom were injured and Justin Morrow was suspended. Marky Delgado, who can play on the backline, was with the U.S. under-20 team.
Centre back Damien Perquis recovered from a calf problem to dress, but only made the bench.
The 23-year-old Simonin, taken ninth overall in the 2015 draft from North Carolina State, played the entire match, won 2-1 by RSL.
NOTES: Vancouver goalie David Ousted was named to the MLS team of the Week after a stellar performance in a 2-1 victory over the Portland Timbers. Toronto FC playmaker Sebastian Giovinco received honourable mention ... FC Dallas and the Seattle Sounders failed to register a shot on target in Saturday's 0-0 tie. The Sounders attempted seven shots but five were off target and two were blocked. Dallas had four shots (three off target and one blocked) ... Colorado's 0-0 draw with Houston on Saturday stretched the Rapids' franchise-record winless streak to 17 games. Colorado also becomes the first MLS team to open the regular season with three straight scoreless draws. The club has not scored in 510 minutes of regular-season play.
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