The Cubs lost leads twice in the game, but Renteria praised Jackson's resiliency.
"It doesn't matter," Renteria said of the blown leads. "He went out there and ground it out. We're still in a limited 'pen situation and he ate up innings for us."
The Cubs added right-handed reliever Blake Parker on Sunday, and the bullpen was still feeling the effects of extra-inning games over first two weeks. An off-day Monday will help.
"After your team scores, you definitely want to come out and have a clean inning, but you just have to be aggressive -- that's a good hitting team," Jackson said. "When you have opportunities to put guys away, you have to put guys away. That's pretty much what it boils down to."
Michael Wacha struck out eight over 6 1/3 innings and picked up the win despite serving up Anthony Rizzo's two-run homer in the first. Rizzo gave the Cubs the lead with one out when he hit his second home run of the season, launching the first pitch from Wacha into the Cardinals' bullpen in right to drive in Justin Ruggiano, who had singled.
Jackson, making his third start, needed 26 pitches to get through the first, and threw another 30 in the second. He walked Allen Craig to open the second and Jhonny Peralta singled. Two outs later, both scored on Matt Carpenter's single to tie the game and Kolten Wong followed with a RBI single.
The problem in the second inning, Jackson said, was not just walking Craig, but that he fell behind too many hitters.
"It's just a matter of making pitches and attacking the strike zone," Jackson said. "If I make those pitches [when the count's 1-2], that definitely changes the outcome of the game."
Yadier Molina singled to open the Cardinals third, and then rain stopped play for 46 minutes. Jackson returned after the delay, and finished the third efficiently, and kept going. After throwing 59 pitches prior to the rain, Jackson needed 55 to get through his last four innings.
Jackson kept his right arm wrapped during the delay, and said the time passed quickly. He didn't throw, didn't change jerseys.
"I just tried to keep the same mind frame and stay ready for whenever the rain delay was over," Jackson said.
He's had to wait an hour during a game in the past.
"He went out there and ground it out," Renteria said. "We did everything we could to keep his arm warm. We knew the window that was available for us. He's a veteran and he's been out there before and I think he's got a will and a strong body and he was able to go out and do it."
Jackson led the National League in losses last season and says he feels better this season.
"I feel good, I feel like I'm in a rhythm," he said. "You come out, make a pitch and get out of the inning clean, and it changes the whole game. From the third inning on, they scored one run [off me]. ... I feel good. I feel I can go out and make them put the ball in play. It's a grind. You continue to battle and keep a positive mind frame. I don't feel anything similar to last year now."
Wacha also stayed in the game after the delay, and with two outs in the Chicago fourth, Junior Lake singled off the right-hander, Mike Olt was hit on the left hand by a pitch, and Welington Castillo delivered an RBI single to tie the game.
Peter Bourjos tripled to lead off the Cardinals fourth and scored one out later on Carpenter's sacrifice fly. Peralta added an RBI double in the eighth, and another run scored from third on Bourjos' fielder's choice. Bourjos hit the ball to third baseman Olt, who bobbled the ball, then threw home, but pulled Castillo off the plate. Olt was charged with an error.
Lake tripled with one out in the ninth and scored on Olt's single off Trevor Rosenthal.
The Cubs may have lost the series but leave St. Louis feeling good about themselves.
"We definitely have a lot of fight in us," Jackson said. "Win, lose or draw, we don't have a group of guys who are going to give up. We're going to go out and battle. We may come up short sometimes, but we're going to go out and give 110 percent on the field. As a team, thats all you want. If we keep fighting like we did today, we're going to win a lot of ballgames."
The Cubs got off to a rough start last April, but have a new manager and some new faces on the roster. Still, does it feel anything like last year?
"I don't think so," Jackson said. "I feel there's a different attitude in here."
Renteria has talked about his team's fighting spirit early in the season, and repeated that Sunday.
"If we keep fighting like that and we dont quit -- [Saturday] was the only game where I could tell you that I thought [they stopped battling] and maybe one game at home against Pittsburgh," Renteria said. "Every day, we're in it to the end."
The Cubs still have yet to win a series this season.
"I think if we keep pushing, at some point, it's got to turn," Renteria said. "If we were playing really bad baseball, I'd go, 'Gosh, I'd be really concerned.' The reality is they're showing a lot of fight, guys are picking each other up, they're going out every single day to try to win a ballgame and today was no different.
"I guarantee you they weren't very comfortable in that [Cardinals] dugout," he said.