PHILADELPHIA -- Taijuan Walker has had two lingering problems this season: His splitter and his velocity.

His ongoing search for both continued in the Phillies' 5-4 series-opening loss to the D-backs on Friday night at Citizens Bank Park. Walker allowed four runs off five hits (including three homers) and three walks over just four innings.

"I know he's working at it and I know he cares," manager Rob Thomson said. "That's why I have patience with him. I'll probably sit down with him [Saturday], just to make sure he's healthy."

Asked about that potential conversation, Walker said: "I feel healthy."

The Phillies could have a difficult decision to make after Spencer Turnbull worked around some traffic to toss three scoreless innings in relief of Walker. Turnbull posted a 1.67 ERA in six starts while Walker was on the IL in April. He limited opponents to a .489 OPS and just three home runs over 32 1/3 innings in those outings.

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Walker has conceded 12 home runs and an .862 opponents' OPS in his 10 starts. Only two of the 10 have been quality starts. His 5.60 ERA ranks 114th out of 125 pitchers with at least 10 starts this season.

Has Thomson considered making a change?

"Even if I did, I wouldn't tell you," Thomson said. "That's an inside-the-clubhouse conversation."

One start after Walker threw only seven splitters -- six fewer than any other outing as a Phillie -- he threw just nine against the D-backs. He abandoned the pitch his last time out due to his inability to locate it, and that was once again the issue on Friday.

Of the nine, only one was thrown below the knees. Four were put in play -- and they were all well struck.

"I threw one good one and then next thing I know, I give up a home run on it and it stayed up from there," Walker said. "Tried to force it down -- get it down -- but the action is just not consistent right now."

Unlike against the O's, Walker was unable to find other pitches to keep the D-backs off balance. He generated only two swings and misses on 26 pitches. The two whiffs were tied for his fewest in any of his 208 career outings, while his whiff rate of 7.7% was his third-lowest -- and his worst since June 3, 2021, with the Mets.

"He struggled, you know?" Thomson said. "Command was off, up in the zone. ... Everything was up in the zone and hittable."

As for Walker's velocity, his sinker topped out at 91.5 mph. It averaged 90.4 mph for the second straight outing, which is down from his season average of 91 mph -- which, in turn, is significantly down from 92.4 mph last season.

“I feel 100% fine,” Walker said. “My arm felt really good today. I thought the ball was coming out good and I look up and it's 91.”

Still, Walker has proven he can be effective at that velocity – but only if the splitter is working.

"If the splitter's there, then I feel like the velocity doesn't play that big of a part in it," Walker said. "Because I'd be throwing my splitter 30-plus percent of the time."

Walker threw his splitter 33.2% of the time last year, when opponents hit just .205 against it. But they're hitting .426 (23-for-54) against it this year.

He's thrown it only 16 times out of 169 pitches (9.5%) in his last two outings.

Walker needs to rediscover his feel for the splitter in order to be successful, but it's hard to do that when he's not throwing it often. At the same time, it's hard to throw it often when opponents are hitting it so hard.

"Should he throw more? Yeah," Thomson said. "But if it's going to be up in the strike zone, then he probably shouldn't."

Walker was far from the Phillies’ only problem on Friday night.

They went 1-for-8 with runners in scoring position and stranded six runners. Bryce Harper grounded into a 1-2-3 double play to end a bases-loaded threat in the seventh. That came one inning after he didn't tag up from second with zero outs on an Alec Bohm fly out that required a leaping catch at the right-field wall.

“We had a couple chances, really,” Thomson said. “Bryce leads off with a double, we don't score. He probably should be tagging on that ball with nobody out -- and he knows that. We had some chances, just didn't come through.”

For Walker, he knows his chances could be running out.

“Obviously, I’m busting my butt,” Walker said. “But I know I've got to clean my [stuff] up.”