Rennie said the pair had suffered cramp - in Afeaki's case in both hamstrings - due mainly to their recent light training loads.
Flanker Sam Cane, who has had a back problem, will also be available to play the Crusaders in what has the potential to be a big occasion at Waikato Stadium.
"They were very strong defensively," Rennie noted of the Crusaders during their comprehensive 23-3 victory over the Blues in Christchurch. "They're starting to get back to near their best. Certainly they've got a very good kicking game and they played at the right end of the field. They can suffocate you down there.
"To keep the Blues tryless was a good effort. We'll have to go pretty well and challenge them in a lot of areas to break down their defence."
In contrast, the Chiefs were forced to scrap for their win in Wellington, although their defence was also excellent as they counted down the final 10 minutes with only 14 men after Ben Tameifuna's sinbinning.
"That's the benchmark of where we need to be," Rennie said. "Getting four points against the Hurricanes was important. It strengthens our position and puts them under a bit more pressure."
The Crusaders have injury issues of their own, although they are minor compared with the Chiefs.
Crusaders hooker Ben Funnell, who received a late call-up to start against the Blues after Corey Flynn's hamstring strain in the warm-up, has suffered a minor knee ligament injury. Todd Blackadder has called in Taranaki hooker Laurence Corlett as cover.
Utility back Adam Whitelock also suffered a knee injury and is in doubt.