Smalling featured for the first time in over a month in Sunday's defeat to Cagliari.
Fonseca said, “He did very well. It was important to get Chris at least 45 minutes of playing time, he had more than that, and I think it was positive. Now he has the chance to play against Manchester United.
“Unfortunately, we were unable to give some playing time to all those who were coming back from injury, but Chris and Leonardo Spinazzola were the most important ones to get minutes in their legs."
Fonseca was asked if he will start Bryan Cristante or Smalling at Old Trafford on Thursday.
“We will see. It is possible we will use them both."