Denorfia, 34, will join the Mariners on Friday when they open a three-game series in Baltimore, along with center fielder Austin Jackson, who was acquired from the Tigers in a separate deal.
"Right now, my plan is to play him and play him often," manager Lloyd McClendon said of Denorfia. "I'm not sure if it'll be a strict platoon. I can't say that. He's pretty adept against right-handers as well. If you look at his numbers, last year he hit .284 against lefties, .276 against righties. He's a professional hitter with a nice track record."
Denorfia has hit .275/.335/.397 over parts of nine seasons with the Reds, A's and Padres. Denorfia is earning $2.25 million in 2014 and will be a free agent after this season.
Denorfia had a .311/.378/.471 line against left-handed pitching with the Padres from 2010-13. His production against lefties has dipped this season (.253/.311/.326), though he could help a Mariners team that has had an all left-handed hitting outfield of Dustin Ackley, James Jones and Endy Chavez in recent weeks.
"When you can put guys like that in the lineup, it gives you presence," McClendon said. "It gives your lineup respectability. I know [Kendrys] Morales hasn't hit yet, but he gives us presence and he's going to hit. Denorfia is going to hit. Your lineup becomes more professional with guys in it that are capable of driving in a big run.
"Down the stretch, that's what we're going to need because we're going to face good pitching. Everybody is going to have good pitching. You need professional hitters."
The 6-foot, 195-pounder hit .242 with one home run and 16 RBIs in 248 at-bats for San Diego this season.
"These situations usually give you a shot in the arm," McClendon said. "Hopefully that's the case here. He's excited about coming here with a chance to win and we'll see what happens."
Almonte, 25, opened the season as Seattle's starting center fielder, but was sent down after batting .198 with one home run and eight RBIs in 27 games. He's hit .267/.333./.390 with six home runs and 31 RBIs in 72 games for Triple-A Tacoma.
Kohlscheen, 25, is 3-1 with a 2.70 ERA in 38 appearances while splitting the year between Tacoma and Double-A Jackson.