HOUSTON — In the Houston Astros’ biggest game of the season, first base belonged to backup catcher Victor Caratini, furthering questions about Christian Walker’s future as an everyday player.

Walker has underperformed in the first season of a three-year, $60 million contract he signed this offseason, forcing manager Joe Espada to gradually move him down in Houston’s batting order. Walker, the team’s Opening Day cleanup hitter, batted seventh thrice this month.

Friday’s initial lineup had Walker back in the seven-hole, but the club moved him up to sixth shortly before the game began. Walker struck out three times anyway in the 4-0 loss against the Seattle Mariners.

On Saturday, with Seattle holding a one-game American League West lead over the Astros and in position to secure the season series, Walker did not start at all. Asked whether he was surprised, Walker replied: “Yeah, I expect to play every day.”

“But, better for the team (to) get another lefty in there. I’ve been grinding at home. It is what it is.”

Walker is slashing .202/.281/.345 in 285 plate appearances at Daikin Park. Fifteen of his 23 home runs have come on the road, where he is slugging .446 with a .753 OPS. Walker sat on Saturday mired in a 15-for-75 funk, part of a forgettable first season that must prompt some deeper questions.

That Espada did not start him in the most significant game of Houston’s season only deepens them. The second-year manager downplayed the decision, citing the balance a switch-hitting Caratini will bring to the lineup. Houston had a team off day on Thursday, too, so the decision isn’t workload-related.

Espada interrupted a question that wondered whether first base will be an open job for the remaining eight regular-season games. “No, no, no, no,” Espada said.

“This is just me getting Caratini, a lefty bat, in there,” Espada added. “Walker has been hot. He’s been our first baseman. He’ll continue to be our first baseman.”

According to Walker, Espada told him that “he wanted to work another lefty in.”

“He’s the manager,” Walker said. “He’s in control. Trust what he’s doing.”

Walker entered the day with a .696 OPS. Of the 25 qualified hitters with a lower OPS, only Atlanta’s Ozzie Albies and Michael Harris II had taken more plate appearances than Walker’s 614.

Walker does lead the Astros with 82 RBIs, but he has 89 strikeouts with runners on base this season. No hitter in the sport has more, making it almost impossible for Espada to rationalize hitting him near the middle of the batting order. The lack of other viable alternatives has forced Espada to do so anyway.

A brutal beginning to Walker’s season has altered his overall numbers, but he’s been productive since the All-Star break, slashing .257/.307/.446 across 237 plate appearances. Walker finished last season with a .251/.335/.468 slash line. His 23 home runs this season are three fewer than his total last year.

A career-high 172 strikeouts have sunk Walker’s on-base percentage, as has his inability to hit four-seam fastballs. Walker is batting just .211 against four-seamers and, according to Baseball Savant, has a minus-2 run value against the pitch. Last season, it was plus-9.

“All things considered where I started, I’m proud of the season and how it’s gone,” Walker said. “I think it would’ve been easy to just roll over and accept a s—- year for what it is, but to get the homer totals up in the 20s — the average is hard to move once the bulk gets high enough — but all things considered, it’s not the year I planned but I’m still proud of what I threw out there.”

Whether that is enough to eliminate these questions — both now and into the winter — is a mystery. A crowded infield will force some fascinating conversations this offseason. Carlos Correa is cemented as the team’s third baseman over Isaac Paredes, who just returned from a significant right hamstring strain.

Paredes has experience playing first base, making Walker a logical candidate to be discussed in possible trades. His contract includes a limited no-trade provision, which would give him the authority to block trades to six teams.

Given his poor production, the Astros likely must eat at least some of the $40 million remaining on Walker’s contract should they choose to shop him. Walker is well-liked in the Astros’ clubhouse and has been a consummate professional in the face of his pronounced struggles.

Reversing them during the team’s biggest stretch of the season would do wonders to calm some of the speculation about his future.

(Photo of Christian Walker during an at-bat earlier this season: Alex Slitz / Getty Images)