Two days on from beating Frances Tiafoe in a three-set epic, Murray fell to a 6-4 7-6 (8-6) loss to the No.2 seed from Argentina in Antwerp.
The three-time grand slam winner, who in January 2019 underwent hip resurfacing surgery, was 4-1 up in the first set but was reeled back in and lost out in a hard-fought contest.
After the defeat, a frustrated Murray said: "I didn't make as many good decisions as I would have liked in the second set dealing with adversity.
"Mentally I was poor, and my attitude was poor on the court."
Murray is still slated to play in Austria and Stockholm before the season is out, while he could also enter the Paris Masters either via a wildcard entry or going through the qualifiers.
"There'll be a decision on the final Paris wildcard on Monday, but I might even play the qualis there," Murray added.
"Sport is a results business. Play well or poorly doesn't really matter if you lose matches. You need to be winning.
"That's what I want in the last few tournaments. They are really strong tournaments and there are no guarantees the results will come, but I want to win more matches."
No.7 seed Lloyd Harris is also into the quarter-finals after a straightforward 6-2 6-3 victory over Jan-Lennard Struff in just 72 minutes.
Marton Fucsovics came through 6-7 (5-7) 6-3 6-1 against Roberto Bautista Agut and Brandon Nakashima edged past Henri Laaksonen 7-6 (7-5) 6-7 (5-7) 6-3 to earn the right to face Schwartzman.