Sizemore ran the bases for the second time in three days Tuesday, 10 months after having microfracture surgery on his left knee.
"I watched him going after fly balls and it was very impressive," Acta said. "He was running, going after them."
Sizemore hurt his knee in an exhibition game April 1 last year. He played in 33 regular-season games before re-aggravating the injury while sliding. He went on the disabled list May 19 and had surgery June 4.
The three-time American League All-Star also missed the last part of the 2009 season following surgery on his left elbow.