The Leinster flanker felt for Wallace, as his dreams of playing in New Zealand were dashed by the knee ligament damage he sustained in last weekend's defeat to England.
"On Saturday I was watching the game and when Wally went down I didn't know what to think," explained Jennings, who made his Ireland debut some three months before the 2007 World Cup.
"Initially your thoughts go out to him because it was a bad bang he took, but I knew I was on the standby list. Sport can be very cruel sometimes.
"I was told before the France game that I wasn't going to be involved, so it was a matter of getting my head around it. It was obviously a huge disappointment (to miss out on selection).
"And then I got back into Leinster and was focusing on the first six league games before we kind of break up from there. We played on Friday night against Northampton and I played a half.
"Then after Wally got injured, I was fortunate I got the call that night and the plans that you'd made and the way things were going to be looking for the next couple of weeks changed like that.
"Someone's misery is another person's joy and it was very unfortunate for Wally but very fortunate for me.
"It's obviously not the way you would want things to happen, but these things happen and you've got to make the most of it and I'm certainly going to try and do that and enjoy it."
Readily admitting that he is here to 'get the number 7 jersey', his prime focus is on playing his way into the Irish starting line-up. He only got a 60-minute runout against France at the Aviva Stadium last month.
The 30-year-old broke his arm in the Magners League grand final and had it operated on in June. At the time, some thought he might not make it back in time for the World Cup.
But the determination that has been the hallmark of a career that includes successful spells with both Leinster and Leicester Tigers got him back into contention.
Jennings clearly thrives in a winning environment and he is itching to make the most of his opportunity at the World Cup, with his 10 Irish caps to date won over a four-year span.
Asked about the competition for places in the squad, he insisted: "It's not just me. We've got to have 30 lads here pushing hard for these spots because if we don't and we have lads pushing along and holding tackle bags and happy to not compete for places, we're going to come up short.
"It's the same whether you're with your province or your club or whether you're here, we've got to give headaches to Declan (Kidney) and the coaches and be pushing each other.
"We've got to be pushing each other hard for this USA game and the 15 guys who get out there have got to do a good job. There'll be pressure on them if they don't perform because there's going to be changes.
"It's certainly my intention to push very, very hard for the place and I know everybody else in the squad, the 30 lads, are doing the same and hopefully it will benefit us all."
As a noted leader and a specialist openside with hardened skills at the breakdown, Jennings feels he can offer different qualities to some of the other back rowers in the squad.
"I'm certainly not a ball carrier like Wally or Seanie (O'Brien). I try to get as much ball as possible and link up. I'm a different player to the other guys in the back row."
His provincial head coach Joe Schmidt lost a high calibre player for the opening weeks of the season when Jennings got his late call-up, but he is nonethless delighted that the Dubliner is Down Under with the rest of the world's best players.
"I think what Jenno's achieved he's deserving of it. But I wouldn't say a word against David Wallace. I think he's a fabulous player. This was likely his last opportunity to play in a World Cup and I think that's pretty devastating for Wally," conceded Schmidt.
"I think Jenno is deceptively powerful. He's 105 kilos. He's not a small man by any means. That doesn't make him massive but it certainly makes him able to fight his corner.
"He tends to get in and put guys on the ground very quickly. He doesn't really necessarily wrestle them up top. He puts them on the ground.
"For Jenno, once you've got them on the ground the power game comes out of it a little bit. If you are really efficient at doing that I think that enables him to be really effective.
"When he carries he has a few subtle skills, nice offloads. I still remember his chip and chase against Saracens at Wembley. I thought that was pretty impressive.
"He's got a subtle skills set. Possibly people would see that, maybe under closer examination."
Jennings wants to make the most of his opportunity in New Zealand and he can sense the start of something big for this squad, with the focus squarely on beating the USA in their Pool C opener next weekend.
"We had a meeting on the Tuesday before we set off for New Zealand and there was a buzz around the room," he added.
"Unfortunately results haven't gone our way. We have to accept that and work on those things. It's just a matter of putting that behind us now. We are here to do a job.
"Pre-season games only account for so much. It's all about how we perform in the World Cup and that starts against the Americans."