Last weekend's 3-1 league defeat at Udinese was Inter's first in six games under the Brazilian coach, who replaced Rafael Benitez on December 24 after last term's treble winners had endured a difficult first half of the campaign.
Wednesday's fortunate Italian Cup quarter-final win at Napoli was thanks to a penalty shootout after a poor attacking display from Inter in the 0-0 draw, where the hosts peppered the goal for long spells.
Injuries to Dejan Stankovic and Ivan Cordoba took some of the gloss off the win but the pair are suspended for the Palermo game anyway, as is left back Cristian Chivu.
"We must keep in mind that the side is adapting to the change in coach and this has a bearing on performances," said Leonardo, whose fifth-placed side trail leaders and his former club AC Milan by nine points.
Striker Diego Milito may return from a minor thigh problem and last season's club top scorer is badly needed with deputy Goran Pandev looking utterly unconvincing in front of goal.
His problems underline why Inter want another striker before the end of the January transfer window with Sampdoria's Giampaolo Pazzini the favourite to join after Samp president Riccardo Garrone said "we will see" when asked about a deal.
"If we can add a player it will be an improvement," Leonardo said.
Milan have already been busy in the transfer window with Antonio Cassano and Dutch duo Mark van Bommel and Urby Emanuelson arriving.
The latter two made impressive debuts in midfield in Wednesday's 2-1 Italian Cup win at Sampdoria while Cassano has produced several magical passes in recent weeks.
Milan, though, are not firing on all cylinders at the moment and visit Catania on Saturday in the Sicilians' first home outing under new uncompromising coach Diego Simeone.
Their quirky Massimino ground is a tough place to go at any time and Milan's long injury list adds to the difficulty.
Second-placed Napoli, four points behind Milan, entertain Sampdoria on Sunday while AS Roma in third tackle improving Bologna.
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