According to philly.com, Nola threw three "up and down" sets of 15 pitches on Monday. The sets were meant to simulate a game in which a pitcher would have breaks between each set of 15 pitches.
Mackanin said on Tuesday that Nola showed improvement but that the Phillies still plan to err on the side of caution before determining their next move.
Nola is 2-0 in three appearances this season, with a 4.50 ERA. His last outing was on April 20, when he earned the win after giving up four runs on seven hits against the Mets. He struck out two and walked four in five innings.
Nola's rotation spot comes up on Friday, against the Nationals, but Mackanin said on Tuesday that the Phillies may need another day to decide if Nola will make a Minor League rehab start before he returns to the rotation.
"He showed improvement [on Monday], but we're not sure we're ready to throw him back out there," Mackanin said before the game against the Cubs.
Nola experienced pain during a throwing session last Friday in Los Angeles; Monday's bullpen session went much better.
"It still felt a little sore from the last time, but it felt a lot better," Nola said on Monday. "If it was hurting like it did the other day, I couldn't have done three ups and downs.
"The third one felt the best of all, so that's a good sign."
Jeff Arnold is a contributor to MLB.com based in Chicago and covered the Phillies on Tuesday. This story was not subject to the approval of Major League Baseball or its clubs.