The Dutch club returned to the field for the first time since the incident with a closed-doors friendly against Belgian club Racing Genk on Saturday, having spent the last week dealing with the trauma of Nouri's collapse and diagnosis.
"It has been a week of emotion and sadness and supporting his family," Keizer told Dutch television.
"We were on the training field on Thursday when the team manager asked us to stop and go upstairs for a meeting. That was already a sign and then we were told the bad news.
"We've been thinking the whole time of 'Appie'. It's really difficult, at times we all have our weak moments. Luckily, thanks to the football, we can dull those senses a bit.
"I think the players' enjoyment is coming back, even if it takes a while. It will be different for each individual. But we must make sure we focus ourselves to play for important matters."
Ajax said on Thursday that Nouri's brain damage is "serious and permanent" and he has no chance of a full recovery after the 20-year-old collapsed during a pre-season friendly against Werder Bremen in Hippach, Austria on July 8.
He was revived on the pitch and airlifted to hospital but Ajax confirmed that although his heart is working normally, he has lost function in parts of his brain.
"The chances of recovery of these crucial brain functions is nil," the club said in a statement.
"This was probably due to a lack of oxygen supply to his brain." Nouri, a Dutch youth international born in Amsterdam to parents of Moroccan descent, made 15 appearances for the Dutch club last season, including three in the Europa League, where Ajax reached the final.
Ajax play Nice on July 25 in the third qualifying round of the Champions League.
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