Junior Etou's three-point play with 40 seconds left put Rutgers ahead for good, 68-66.
"It was a good feeling coming back, but not a good feeling when you don't get the win," DeCosey said.
Kadeem Jack scored 18 points to lead the Scarlet Knights (7-7).
DeCosey had 25 points but cooled off late in the second half. He seemed like a one-man band for the Owls until Dalton Pepper suddenly joined in.
DeCosey had 25 of Temple's first 42 points, but the Owls trailed by their largest margin, 49-42, with a little less than 11 minutes remaining.
That's when Pepper took over. He scored the next 19 points for Temple over a stretch of six-plus minutes to keep the Owls alive.
Pepper finished with 19 points.
DeCosey didn't miss a field goal in his stellar first half until less than a minute remained. He had 17 points in the first half, shooting 6 for 7 from the field and 4 for 4 from three-point range.
DeCosey hadn't made more than three three-pointers in a game all season.
He made his first six Wednesday before missing a desperation three at the final buzzer.
Temple led by as many as eight in the first half, but Rutgers closed the half on a 13-6 run and tied it at 30.
"Twenty-four baskets and only nine assists. We weren't moving the ball well enough," Owls coach Fran Dunphy said. "We tried to take it in our own hands too many times."
Temple was playing its first game without key reserve Daniel Dingle. The sophomore forward and sixth man is likely to miss the rest of the season with a meniscus tear in his right knee.
Temple and Rutgers played 13 years together in the Atlantic Ten Conference, ending in 1995.
Their time together in the AAC will be a short one. Rutgers is leaving the new conference for the Big Ten next season.