With his side 1-0 down at Anfield, Martial passed up a great opportunity to bring the visitors back into the game in the second half.
The France international played a one-two with Andreas Pereira and was sent through on goal, only for his shot to miss the target.
It proved to be United's best chance and Mohamed Salah went on to add a second in the late stages of the game as Liverpool extended their lead at the top of the Premier League to 16 points.
And ex-United star Keane believes the missed chance shows that the France international, who has eight goals from 16 games in the English top-flight this season, is not at the level the Old Trafford side need.
"Anthony Martial has come to Manchester United and the big strikers score in the big moments, but his miss sums up his career at the club - excellent build-up play, but you have to hit the target," he said on Sky Sports.
"No excuses. The big strikers who are remembered by the supporters hit the target and the back of the net, and that’s why that guy is not quite good enough for Manchester United.
"That moment summed him up."
United are low on attacking options after seeing Marcus Rashford ruled out for at least six weeks with a back injury.
Red Devils manager Ole Gunnar Solskjaer suggested after Sunday's game that they could be forced to look for a short-term replacement in the transfer market this month.
“We've had many injuries for big players this season. It’s just an unfortunate situation we're in," he told Sky Sports. "We might look at some short term deals as well that could take us through to the summer. We don’t desperately need a striker, if the right one is there and it fits for us... we’ve got players who are champing at the bit.”