The 2003 National League Rookie of the Year had originally retired from baseball in July.
Willis, who turns 31 next week, last pitched professionally for the Baltimore Orioles' Triple-A affiliate in 2012. He gave up eight runs in only 6.1 innings pitched and subsequently retired.
Ever since leaving the Florida Marlins—who he last pitched for in 2007—Willis has suffered from constant injuries and has struggled to regain form.
The two-time All-Star, however, was lightning earlier in his career.
Willis led the league with 22 wins, seven complete games and five shutouts in 2005 while finishing second in Cy Young Award voting. He also led the league with 35 games started in 2007—though he also was the leader in earned runs as well.
Though Willis' current mindset seems to be focused on returning to baseball, he's also no stranger to erratic behavior—as well as erratic pitching.
After his horrific outing with the the Orioles' minor league affiliate last year, Willis was placed on the restricted list for two months and later filed a grievance against the ball club after it demoted him to the bullpen.
He also hasn't had anything lower than a 4.98 ERA since 2008 and has averaged 8.2 walks per nine innings in that span.
Has his short time away from the game given him enough time to clear his head and focus on pitching?
Whether or not it has, this decision comes with little risk to the Cubs and could pay dividends if Willis can show a glimmer of his prior self.
What do you think? Will he be able to come back from five years of obscurity?