Simone Inzaghi could not assure Romelu Lukaku he would start again against Porto — despite ending his goal drought on Saturday.
Lukaku has scored only three times since returning to Inter MIlan at the start of this season.
It has been a difficult campaign for the Belgium forward, who also failed to net at the World Cup in Qatar.
But a penalty against Udinese on Saturday gave Lukaku his first Serie A goal since August ahead of Wednesday's Champions League last-16 first leg against Porto.
That does not mean Lukaku is certain to start, however, as coach Inzaghi said in his news conference there were "doubts" about his line-up.
Edin Dzeko would be the obvious alternative to Lukaku, having netted 11 in all competitions this season, although the pair started together against Udinese.
Asked specifically about Lukaku and midfielder Marcelo Brozovic, Inzaghi said: "Lukaku has been working for longer and is improving. He is putting a great deal of effort into it.
"As for both, tomorrow we will try to make the best choices for Inter.
"Those who do not start will still be useful in the game. In games like these, the final minutes are those in which decisive actions can occur and spaces are freed up to exploit."
Inter have not been past the last 16 of the Champions League since the 2010-11 season, when they were playing as defending champions.
But having emerged from a difficult group that also included Barcelona and Bayern Munich, confidence is high.
"We face this tie with great confidence," Inzaghi said. "We reached it with an incredible path – on paper, Barcelona and Bayern Munich were teams that everyone wanted to avoid.
"You can see what Barca are doing in La Liga, and we know what Bayern are."
