The Dutch striker has battled for fitness this campaign, making just four appearances, and will now focus on getting himself 100% for the 2014 season, provided he remains with the Canadian club.
"Danny is probably out for the season," Nelsen told MLSsoccer.com. "He has a calf injury and it is going to be tight and hard for him [to get fit again before the season ends].
"He unfortunately just can't get over that last hurdle. It is a shame and I feel bad for Danny. I also feel bad for everybody because we really needed him."
He added: "I think we'll sit down at the end of the season and talk to him."