He was amazed to receive a telephone call from the police last month asking him to do an E-fit - a computer-enhanced impression.
But Roy Westbrook told the public inquiry into Stephen Lawrence's death that, after speaking to an expert at Scotland Yard, he was told it was too late to be any good.
Mr Westbrook said he was told his description was now likely to be "recognition rather than recollection".
He said he was not approached in 1993 when he saw a gang of white youths stab the black 18-year-old student near a bus stop in Eltham, south London.
But a few hours after being told he would be needed as a witness to the public inquiry in London, Mr Westbrook, a support worker for people with learning difficulties, said he had a number of messages about a photofit on his answering machine.
"I found it very puzzling" he told the inquiry in south London. "I spoke to someone at Scotland Yard about it and was told it would be recognition rather than recollection. It was far too late."
Mr Westbrook said he saw Stephen Lawrence and his friend Duwane Brooks being chased along the road by a group of white youths.
They surrounded Stephen and attacked him.
"He was swallowed up by the weight of the boys and forced to the ground," he added.
A man standing at the bus stop had commented that "he got a good pasting, didn't he? It was probably because he was black".
He was later asked to go to identity parades but left after waiting at the police station for nine hours.
He and other witnesses had been left for long periods to talk to each other and a woman became flustered after being asked for her address.
Another witness, Joseph Shepherd, the inquiry was told, went to one identity parade but had refused to do any others after an inspector called out his name.
Inspector Laurence Slonesaid he used the name to identify the witness. He said: "It was behind closed doors and it was in a conversational voice. If the suspect did hear the name, then I regret that."
No one has been convicted of Stephen Lawrence's murder. Neil Acourt, 21, Gary Dobson, 21, and Luke Knight, 20, were cleared of the murder at the Old Bailey in 1996. The case against David Norris, 20, and Jamie Acourt, 19, never came to trial.
The inquiry will sit again on Tuesday.