But the Whitecaps coach says he has the depth to ensure his team won't skip a beat.
Captain Jay DeMerit, midfielder Gershon Koffie, winger Dane Richards and striker Darren Mattocks will be among the players who were in the lineup for Vancouver's impressive 2-1 win over Western Conference rivals Real Salt Lake last weekend but will miss Wednesday's game.
Still, Rennie expects his lineup to look even better. Leeds United centre-back Andy O'Brien and Scottish international striker Kenny Miller are set to earn their first starting berths for the club against visiting Dallas.
"It's not too many changes really," he said. "It's just a couple of guys coming in from the last game, so it's arguably a stronger team than the one before so I've no concerns about it."
DeMerit had to be substituted in Saturday's win over Salt Lake after suffering a knock to the head in the 40th minute and picking up a concussion.
DeMerit could return a the road game against Seattle Saturday, but meanwhile Scottish midfielder Barry Robson will wear the captain's armband in his absence.
Koffie is suspended while Richards and striker Mattocks are on international duty with Jamaica.
Little-used defender Carlyle Mitchell (Trinidad and Tobago), teenager Russell Teibert (Canada) and 30-year-old striker Etienne Barbara (Malta) will also be representing their respective countries.
However, the Whitecaps (10-7-7) feel they're in good hands against visiting Dallas (6-11-8). O'Brien came off the bench to replace the dazed DeMerit against Real and looked comfortable alongside fellow centre-half Martin Bonjour, impressing returning first-choice goalkeeper Joe Cannon.
"Already I feel he's probably more comfortable with his feet than maybe Jay or Martin, so he'll bring that to us," Cannon said. "He stepped right in the other day.
"But I think with any position on our team right now, it's that way. A guy could go down and we could replace him with someone and we don't lose much if anything at all."
Cannon's praise of his team's depth was echoed by experienced midfielder John Thorrington, who could make his first appearance for the Whitecaps since injuring his right quad in a 3-0 road loss to the Los Angeles Galaxy on June 23.
"I think the organization has done a great job building a strong squad so that when there are international call-ups, injuries, dips of form or whatever it might be, we've got guys ready to step in," he said. "They've built a squad as deep as any I've been a part of."
Thorrington said he was feeling "no ill effects" from the quad strain and could emerge as a replacement for Richards in the starting lineup.
"I've been feeling stronger as each training session comes and if given an opportunity I now feel ready to go," he said.
Of course, the ultimate decision lies with Rennie.
"(Thorrington's) naturally a very fit guy and also a very experienced player," Rennie said. "And he's been one of the best midfielders in the MLS almost every time he's been in a run of games, so he would be ready to go if he's called upon for sure."
The Vancouver faithful will be eager to see former Rangers and Celtic star Miller have an impact in his first start for the club, but Rennie is warning against expecting too much.
"We wouldn't be putting too much pressure on him to do anything spectacular," he said, adding the 32-year-old was still in pre-season mode. "But I think he'll contribute well."
Dallas has only won three times in its past 19 games and sits 10 points outside a playoff spot.
The Toros have a miserable road record — one win in 11 games — and will find it even harder to cause an upset with midfield trio Brek Shea (USA), Julian de Guzman (Canada) and Carlos Rodriguez (Panama) also away on international duty.
Rennie has called on MLS to break during national team action.
"It would be nice to have that (break) and I think it would benefit everyone — especially when it comes down the stretch," he said.
Notes: The Vancouver Whitecaps will salute the Canadian women's Olympic soccer team before the game. Sophie Schmidt, Karina LeBlanc and Emily Zurrer will be among the players honoured.