Television pictures showed Eto'o, currently at Everton, arriving at a clinic in Rome for a medical. He was due to be presented to the crowd before Sunday's Serie A match at home to Palermo.
But Mihajlovic said he did not want his players to be carried away by the expected festive atmosphere.
"Party, what party?" he told a news conference. "Eto'o is joining this team but he must not be a distraction.
"He's a great player, who has won everything and that attracts a lot of attention from the media, but this should not distract us.
"The reason we are (joint) third is thanks to these guys, their commitment and their daily work."
Eto'o, 33, who is a free agent, joined Everton from Chelsea in August after one year at Stamford Bridge. In the same month, he announced his international retirement after winning 118 caps and scoring a record 56 goals for Cameroon.
Eto'o has won four league titles, seven domestic cups, a FIFA Club World Cup medal and three Champions League, two with Barcelona and one with Inter Milan.
Sampdoria are joint third in Serie A alongside Napoli with 33 points from 19 games.
(Writing by Brian Homewood in Berne, editing by Alan Baldwin)
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