Everton defender Ben Godfrey has been taken to hospital to have a lower leg injury assessed which he suffered in Saturday's defeat by Chelsea.
The 24-year-old picked up the injury after making an important tackle on Chelsea's Kai Havertz in the 10th minute of Everton's Premier League opener.
He received treatment on the pitch and was then stretchered off, before being taken to hospital to have his lower leg assessed.
Fibula injuries can take anything from six weeks to three months to recover from depending on the severity.
Fellow centre-back Yerry Mina also went off with an ankle injury in the second half.
On Godfrey's injury, Lampard told Sky Sports: "We think it is a small fracture of his fibula, so we will see how long that will be."
On Mina, the Everton boss said: "It is an ankle injury which we don't know much about at the moment.
"They are freak injuries and we have to make sure the squad is strong enough to handle those things. We are working on that (transfers) and there is movement."
Everton are expected to complete the signing Belgian midfielder Amadou Onana, who watched the match from the stand, in the coming days. Lampard said: "(He brings) a lot. I am a big fan, that is why he is coming.
"When you come in as a manager you want to influence the style of the team and he is a young player that has, for me, everything in terms of physicality, quality on the ball and defensive mindset, an all-rounder for us.
"He is going to have a big impact on the club and is a good lad with real confidence about him. I don't want to hang too much on him, but I am very pleased he is coming in."
On the defeat to Chelsea, Lampard said: "I am frustrated maybe, but not in terms of our performance.
"We gave the lads a solid game plan to defend because Chelsea want to pull you apart and play through. We never allowed them to, other than one tiny moment…and it decided the game.
"I thought we had better chances and our lads gave everything. The energy in our team was brilliant and I thought we deserved a point."