The MLS outfit have won both their matches in the continental competition, the Timbers winning five of their past nine in all competitions.
Portland have also enjoyed great goal-scoring prowess in Group Five, scoring four goals in big wins over United and leaders Olimpia.
The Timbers trail Olimpia on goal difference, but their game in hand against cellar-dwellers Alpha - who they thrashed 4-1 in Guayana in the reverse fixture - is on home soil at Providence Park in Oregon.
Despite some strong form, the Timbers were thrilled to keep a clean sheet in the MLS as they downed the Vancouver Whitecaps 3-0 on Saturday - their first clean sheet in four matches.
Portland defender Liam Ridgewell said the strong defensive performance - their only two clean sheets in their past eight matches have come against the Whitecaps - was a confidence booster.
"Huge. It's big. This is what we've been looking towards," Ridgewell told the Timbers' official website.
"We got a clean sheet a couple of weeks back and we wanted to build off that.
"I think this week we showed what we can do, to build on that 1-0 lead in the first half."
Also on Tuesday, MLS outfit Sporting Kansas City host Nicaragua's Real Esteli.
Sporting KC top Group Two with four points, although an Esteli victory could see the visitors go from last to first in the three-team group.
On Wednesday, Liga MX side Pachuca are at home to Guatemalan outfit Municipal, MLS club DC United welcome Panama's Tauro, New York Red Bulls are away at Honduras against FAS, and Costa Rica's Herediano and Mexican club Leon go head-to-head.
Puerto Rican strugglers Bayamon FC will look to improve on their woeful defensive record - they have shipped 21 goals in three in the competition - when they host Comunicaciones of Guatemala on Thursday.
Also on Thursday, Panama's Chorrillo host Alajuelense of Costa Rica.
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