According to Vimal, Saina had to be administered a dexamethasone injection after the opening round match to curb the pain but it didn't have desired results.
In one of the biggest upsets for India's sagging medal hopes, Saina was knocked out of the Games after suffering an upset 18-21 19-21 loss to World No. 61 Maria Ulitina of Ukraine in 39 minutes in her second Group G match here.
Heartbroken to see his ward lose due to an injury, Vimal said: "I feel very sad for her as she did put in her best efforts in her preparations for this event. Sports at times is very cruel."
Asked how did she suffered the injury, Vimal said: "This comes when you are training hard regularly. Unfortunately for Saina she developed this towards the last week."
The former national coach said Saina had to take a dexamethasone injection to curb the pain, but none in the media covering the Games was aware of this development.
"Saina developed more pain on her knee after her first round match and had been administered dexamethasone injection on that day. This is to bring down the inflammation and pain. She has fat pad inflammation of the knee and has been taking treatment one week prior to our departure," he said.
"She herself decided to take the injection after consulting the IOC doctors here and they have given her that in the Olympic village clinic.
"All the team members are aware of it and not any one in the media. Hoped she would be getting relief from pain today but did not get the desired relief while the injection was taking effect."