Harrell would be fully rested, as his last start came on Tuesday with six scoreless innings against the Twins. But the Rangers have not said for sure if Harrell will be the guy.
"I don't think we've made an official announcement," Rangers manager Jeff Banister said. "I would think he's in line to start if we need him to."
Alvarez joined the Rangers on Thursday, but Harrell has not yet reported. He is expected to report on Friday and will throw a bullpen session. At that point, the Rangers will likely decide if he'll start on Sunday, but it's clear they are continuing trade discussions with other teams to address their pitching needs.
The Rangers' interest in Rays pitchers Chris Archer, Matt Moore and Jake Odorizzi is well known, along with Andrew Cashner of the Padres and Jeremy Hellickson of the Phillies. But acquiring Harrell, who had made just five starts for the Braves since being called up from Triple-A, shows the Rangers are looking at every possibility.
Alvarez, 27, gives the Rangers a second power left-hander out of the bullpen behind Jake Diekman. He made 16 relief appearances for the Braves and had a 3.00 ERA, a 1.07 WHIP and 28 strikeouts in 15 innings.
He had a 12.60 ERA in 10 appearances with the Mets in 2014-15 and a 9.98 ERA this season at Triple-A Las Vegas before being claimed off waivers by the Braves on May 25.
"I'm excited, coming from a last place team to a team competing for first place," Alvarez said. "I'll help the team any way I can."
• Pitcher Derek Holland, on the disabled list with left shoulder inflammation, is scheduled to throw a bullpen session on Friday and live batting practice on Tuesday in Baltimore. He is hoping for an Aug. 20 return.
• Outfielder Shin-Soo Choo, on the disabled list with tightness in his lower back, is expected to hit off Holland on Tuesday. That could tell if he is ready to come off Thursday, the first day he is eligible.
• Left-handed reliever Cesar Ramos has cleared unconditional release waivers and is now a free agent.
T.R. Sullivan has covered the Rangers since 1989, and for MLB.com since 2006. Follow him on Twitter @Sullivan_Ranger and listen to his podcast. This story was not subject to the approval of Major League Baseball or its clubs.