The man who presented Agar with his baggy green cap says the the writing was on the wall for Australia's prospects over the 10 home and away Tests against England when the stuttering batting order had collapsed to 9-117.
No.11 Agar, a teenager on his Test debut, then walked out and smashed 98 from 101 balls.
He combined with Phil Hughes (81no) for a Test record 163-run final-wicket stand, the pair turning a 98-run deficit into a 65-run first innings lead.
Although Agar may not have turned the Test enough for Australia to win, one of the all-time proudest No.11s, says cricket's latest sensation has changed the tone of the two series.
"I think for this whole series, for the whole 10 Tests, it was needed," McGrath told AAP.
"It was looking a little bit embarrassing for the Aussies unfortunately and at 9-117 I thought this could be ugly and set the tone for the rest of the series.
"But that innings with him and Hughes ... brought us back into the game.
"Now we've got a contest here."
Fast bowling legend McGrath held the previous Australian record for the highest individual score for a No.11 with 61 against New Zealand in 2004.
Agar powered beyond that mark and ultimately surpassed the overall record of West Indian Tino Best, who made 95 batting 11 against England last year.
The 19-year-old was one of the biggest selection shocks in Test history, with the secret only out of the bag when McGrath was seen presenting him with his cap on the field at Trent Bridge, shortly before play on Wednesday.
McGrath had never met Agar before, but was immediately impressed.
"I looked up his stats and noticed he was born one month before I made my debut for Australia (1993). So that made me feel a little bit old," he said.
"All I said to him is it's a very proud moment and an honour.
"... And whatever the situation, have that mental strength and never give up because you never know.
"And he's gone out there with no nerves, and looked a very confident young fella.
"And that's what I like about him."