Stefano Pioli struggled to sleep after being told he could return to work with Milan following a negative coronavirus test.
Milan head coach Pioli was unable to be present for the club's past three games after contracting COVID-19 last month, with Daniele Bonera taking charge of first-team affairs in the interim.
The Rossoneri boss and his assistant Giacomo Murelli returned negative results after swabs were taken on Tuesday, clearing him to lead training on Wednesday and return to the dugout for the Europa League Group H meeting with Celtic at San Siro on Thursday.
Milan will advance to the round of 32 if they defeat Celtic and Lille beat Sparta Prague, and the omens are in their favour having suffered just one defeat to the Scottish side in their 11 previous meetings.
"I missed the daily routine so much, being together with the team. When they told me I was negative I struggled to sleep due to the emotion," said Pioli, who will hope to steer his team to a fifth straight success against Celtic.
"Thanks to the club for having improved the technologies that allowed us to work calmly. Thanks to the fans who supported me and thanks to the team, because they made me proud.
"Thanks to Bonera, an intelligent guy. I had COVID but in a fairly light way – I was lucky.
"We did everything necessary to stay close to the team, before and during training as well as in the match, to talk to them after the match and to congratulate them.
"It's better to be present than to stay at home. I can say that 'smart working' doesn't work, because I have worked a lot more from home than on the pitch.
"However, seeing the players live is another thing and that's why I'm happy to be back."
Gianluigi Donnarummas with a fine save #UEL | @acmilan pic.twitter.com/3i7UhmBCdK
— UEFA Europa League (@EuropaLeague) December 2, 2020
Celtic's one point from four games equalled their worst start in the competition since 2009-10, while they have scored just one goal across their previous five visits to Milan.
The Serie A giants were defeated 3-0 by Lille in their previous Europa League game at San Siro and will be hoping to avoid losing back-to-back home matches in the competition for the first time.
