Lacy, a Curtis High School product, looked at the calendar and saw the game against archrival Washington was four weeks away.
"I knew four weeks ago I was going to play this game," Lacy said after helping lead Washington State to a 72-67 win over Washington on Saturday.
In his first game back, Lacy scored 10 points, all in the decisive second half, as the Cougars broke a six-game losing streak to their cross-state rivals.
D.J. Shelton led Washington State (9-12, 2-7 Pac-12) with 20 points and 18 rebounds. Que Johnson added 15 points for the Cougars, who had been in a slump since Lacy was injured.
"We just needed that one game to turn things around," Lacy said. "I think this is that game. We can continue to do that the rest of the season."
Shelton, who added four steals and three assists in 38 minutes, said the return of Lacy loosened up the Washington defense for the rest of the team.
"He opens up so much more for us on the floor," Shelton said.
C.J. Wilcox scored 24 points to lead Washington (13-9, 5-4 Pac-12), which was done in by 38.9 percent shooting in the second half.
"They beat us to the boards," Washington coach Lorenzo Romar said of the Cougars, who won the rebound battle 39-36, including 14 offensive boards. "They shot 20 3-pointers, so with 3-pointers come long rebounds."
The Huskies made just 14 of 36 field goals in the second half, but still were in a position to win as the Cougars shot poorly from the free throw line in the closing minutes.
"No one is out there trying to miss," Romar said. "They just didn't go in. Give them a lot of credit."
Washington State has struggled to score without Lacy as the Cougars are last in the Pac-12 in points per game (62) and field goal percentage (.413).
After a nip-and-tuck first half, the Huskies scored seven unanswered points in the opening minutes of the second for a 34-27 lead, largest of the game to that point. Wilcox had a 3-pointer and layup in that run.
Wilcox hit a 3-pointer to push UW's lead to 42-34.
Washington State fought back with a 12-0 run and took a 46-42 lead with 8:47 left on a 3-pointer that was Lacy's first field goal of the game. Johnson followed with a pair of 3-pointers to push the WSU lead to 52-45.
Baskets by Royce Woolridge and Johnson gave WSU a 56-45 lead with 5 minutes left.
Freshman Nigel Williams-Goss sank a 3-pointer that brought Washington within 59-52 with 2:28 left.
Lacy replied with a 3-pointer for WSU and the Huskies began to foul.
But the Cougars missed three of four late free throws and Williams-Goss hit another 3-pointer to bring UW within 67-63 with 38 seconds left.
Wilcox hit a 3-pointer with 22 seconds left to bring Washington within 70-66. Dexter Kernich-Drew responded by hitting one of two free throws for a 5-point WSU lead.
Perris Blackwell, who had 13 points and 10 rebounds, hit one of two free throws to bring UW within four points. He missed the second and Shelton rebounded and was fouled. He made his first free throw with 6 seconds left but missed his second. The Huskies rebounded but were not able to score as time expired.
Washington State coach Ken Bone said it was great to have Lacy back.
"He's one of those guys who keeps the team together," Bone said.
Shelton has stepped up his play in Lacy's absence, and the senior was huge on Saturday.
"This is his time and he's taking advantage of it," Bone said of Shelton. "He's put in a lot of work."
The first half was close with 12 lead changes. Shelton scored eight points early including two slam dunks and a 3-pointer as Washington State held a 14-13 advantage.
Blackwell completed a 3-point play, breaking a 4-minute UW drought, as the Huskies tied the score at 18 with 8:15 left in the first.
The teams traded baskets the rest of the half. A pair of free throws by Wilcox with a minute left lifted the Huskies to a 27-25 halftime lead.
This marked the 278th meeting between Washington State and Washington, with the Huskies leading the series, 177-101.