Ibrahimovic was ruled out of the Serie A meeting with Hellas Verona on Sunday due to illness and Milan were held to a 1-1 draw at San Siro.
Verona played the last 22 minutes with 10 men after Sofyan Amrabat was shown a red card following a VAR review of his lunge on Samu Castillejo, but the Rossoneri were unable to score a winner.
Capello - who won the Scudetto with the club as a player in 1979 and in the 1990s coached them to four league titles and Champions League success - feels Ibrahimovic is the only player with sufficient character for the club.
"He is an important player, the point of reference for the whole team. When he's there, two defenders are focused on him," he told Radio Anch'Io Sport.
"I saw a timid Milan again at San Siro. When Ibrahimovic is there instead, he does different things.
"But it's not [coach Stefano] Pioli's fault. The players go out with fear and this means they don't have the personality to play for Milan."
Lazio are two points behind Inter, but Capello believes they lack the depth required to compete with Juve and Inter in the long term.
"Inter have added many players to the squad who do not make a difference like the two strikers, but who can help Conte keep things fresh until the end of the season," he said.
"Lazio don't have that. They are two teams that want to try to win and this is a very important thing. Juventus must be very careful."