Four days after seeing their hopes of a sixth successive scudetto crushed in a 3-0 defeat by rivals AC Milan, Inter's hopes of retaining the Champions League were also in tatters after another night of defensive calamity at the San Siro.
Andrea Ranocchia's own goal and Cristian Chivu's second red card in as many games either side of the hour summed up Inter's defensive misfortunes on a dismal night in Milan.
It was the first time a team defending the European Cup have conceded five goals in a match home or away and added to the pressure on coach Leonardo, the second man to occupy the hotspot since Mourinho after Rafa Benitez's sacking in December.
Dejan Stankovic scored an extraordinary goal after less than a minute and Diego Milito, last season's Champions League hero, notched his first in this season's competition as Inter twice led in the opening half.
"Of course, a lot has changed in these four days," he told reporters. "But we have to change this situation... We must stay together and we must try to forget these two heavy defeats.
"The first thing is to give our heads a rest. It's a very mature squad, we need to use our energy on the pitch."
Inter could not have made a better start. With only 54 seconds played, Schalke goalkeeper Manuel Neuer rushed out of his area to head away Esteban Cambiasso's long ball forward only to see his clearance land at the feet of Stankovic.
But two headed chances for Raul showed that Inter's defensive frailties were there to be exposed and it was no surprise when Joel Matip levelled after 17 minutes.
In an end-to-end game, Milito, who scored both goals in last season's 2-0 final win over Bayern Munich, broke his duck this season, side-footing Inter back in front in the 34th minute from fellow Argentine Cambiasso's header.
But Brazilian Edu, little known in his own country, sent Schalke's famously diehard fans into raptures by equalising five minutes before the break, scoring at the second attempt after his better-known compatriot Julio Cesar saved his first effort.
