Walker lasted only two innings and 32 pitches win a 5-0 loss to the Atlanta Braves. He was injured as he ran to cover first base on a ground out to close the inning and came up wincing.
Walker said his back "started getting tight" as he walked back to the dugout. He said he would get an MRI on Wednesday.
"I've never had anything like this before," Walker said. "The training staff isn't too concerned about it, so I'll get an MRI tomorrow and see how I feel."
Walker's injury occurred one day after pitcher Carlos Carrasco suffered a strained left oblique Monday night at Atlanta. The Mets announced Tuesday that Carrasco is expected to miss up to a month in another setback for their rotation.
Carrasco had an MRI on Tuesday that showed a low-grade strain on the left side, the Mets said, adding that a typical timeline for this sort of injury was three to four weeks.
Manager Buck Showalter said Carrasco's diagnosis was actually good news. He feared the injury would be much worse.
"Grade 1, that's good news," Showalter said. "I thought it could be something a little deeper."
Walker, 30, is 10-3 with a 3.85 ERA, while Carrasco, 35, is 13-5 with a 3.92 ERA in a resurgent season for the Mets, who have a 3 1/2-game lead over the Braves in the NL East.
Carrasco gave up three runs in two innings in a 13-1 loss to the Braves. His outing was interrupted by a 55-minute rain delay in the second inning. Carrasco came back after the break and got the last out. He winced on his final pitch and was pulled.
"We're just going to wait to see how I feel tomorrow and go from there," Carrasco said after the game. "This is my first time that I've felt something like this. It just happened on the last pitch of the game."
Carrasco had been 5-0 with a 1.69 ERA in his previous seven starts.
With Carrasco going on the 10-day injured list, the Mets called up right-handers Stephen Nogosek and R.J. Alvarez from Triple A Syracuse. Also, pitcher Adonis Medina was optioned to Syracuse.
"It gives us a couple of fresh arms," Showalter said. "Not fresh, but fresher. Nobody is fresh at this time of year."
In other moves, the Mets designated outfielder Nick Plummer for assignment to make room for Alvarez on the 40-man roster and signed outfielder Travis Jankowski to a minor league contract.
The Associated Press contributed to this report.