Alex Wynter was treated by both medical teams before he was carried off on a stretcher and taken to hospital after colliding with team-mate Tom Eastman.
He had lost consciousness, but Colchester later reported he was sitting up in hospital, under observation, before having scans to determine whether he would be kept in overnight.
Colchester boss Kevin Keen said: "I was very worried for Alex; I'm still worried for Alex.
"I know he's up and aware of what's going on
When he was here he didn't have a clue and was knocked out.
"Head injuries are a big topic
In my family, my son (Eddie Keen, who is in the United States on a football scholarship) has had a head injury which cost him a year of his career.
"It's a big topic and something we've got to look into seriously."
Yet questions may be raised on football's head injury protocols and the decision to allow Eastman to return to the field after receiving stitches to his head wound
He was soon substituted.
"He felt a bit wheezy," Keen added.
Asked whether it was a mistake for Eastman to return to the field, Keen said: "No
It's the FA Cup fourth round, you're playing Tottenham Hotspur at home.
"The boy wants to play
He wants to get on the pitch, so that's what we did.
"Then he said he's not quite right, so we took him off."
Nacer Chadli scored twice to take his tally to four goals in three games, while Eric Dier and Tom Carroll were also on target as Spurs won a fourth straight game.
Ben Davies' own goal was a mere consolation for League One's bottom club, who are without a win in 14 league games.
Keen added: "I thought it was a gallant effort
We had a real go, things didn't go our way, but Tottenham are a top team and it was a tough day."
Spurs had not progressed past this stage since they reached the semi-final in 2012
"The job's done, we're in the next round," Pochettino said.
"When you create a lot of chances and it's impossible to score in the first 15 minutes - we created five, six, seven clear chances - you always invite the opponent to believe.
"But I think we were always in control
Maybe we conceded some chances, but I feel very pleased because of the result."
Pochettino was "very pleased" for Chadli, but believes the Belgian should have had a hat-trick after hitting the crossbar.
"Sometimes you need to keep some goals for the next games," Pochettino said.
Spurs, who play at Norwich on Tuesday, are also in the latter stages of the Europa League and fourth in the Premier League.
Asked if a treble was a realistic target, Pochettino said: "We have quality enough and a strong squad to try to fight for every competition
Then we'll see."
Source : PA