Jones and former Trojans wide receiver Deontay Burnett participated in the special second pro day workout to run the 40 and take part in other drills because hamstring injuries prevented the two players from fully taking part in USC's official pro day on March 21.
Jones, who ran a time of 4.65 in the 40 at the scouting combine in Indianapolis, improved his time to 4.48 on Thursday. Jones said Thursday he estimated his hamstring was 85 percent healed.
Darnold, who was in Cleveland on Wednesday making a visit with the Browns, flew in as a favor to Jones and threw to the running back, even though he already impressed NFL scouts with his performance in the rain on March 21.
Tee Martin, USC's offensive coordinator, told Ohio.com in a telephone interview that Darnold showed what kind of teammate he is.
"When you talk about what kind of guy he is, who does that when you have everything to lose and nothing to gain by doing that?" Martin said. "He had a teammate call him out of the blue. It wasn't something that was planned.
"He gets a call yesterday, and he drops everything else. [Darnold said], 'Yes, I'll be there for you, man,' and gets on an airplane, lands in L.A. this morning, comes over at like 6:30 in the morning and throws for Ronald Jones. I mean, it's crazy. And everyone's like, 'What, Sam?' No one expected him to be here, and he's here because he said, 'Hey, man. My teammate called. He needed me.' So he came. That's unbelievable."
Thursday's workout was attended by 15 NFL teams, USC's official website reported.
Burnett ran his 40 in 4.70 seconds and caught passes from quarterback Troy Williams, who played at Washington in 2015 and at Utah the past two seasons.