Jayden Reid is set for a lengthy spell on the sidelines after suffering a cruciate ligament injury.
The winger only completed his switch to the Blues a week ago, having left Birmingham at the end of last season.
But he was forced off towards the end of Saturday’s pre-season friendly draw at Luton, having been on the pitch for just seven minutes.
Blues boss Danny Cowley said: “Jayden has ruptured his cruciate ligament. We’re waiting for the inflammation to settle down and then the specialist will have a look at it.
“He’ll need to have surgery and is going to be out for the foreseeable future, which is extremely disappointing.
“Jayden came in and did great for us – we were really excited to see how he would develop this season.
“He provides real pace, penetration and athleticism and is also a great person – he really is a lovely lad.
"He’ll need to have surgery and is going to be out for the foreseeable future, which is extremely disappointing."
“As frustrated as we are, however, it pales into insignificance compared to how he’ll be feeling right now.
“But success doesn’t always come in a straight line and I believe Jayden has the character to come through this.
“We’ll get behind him and make sure he receives the best support we can offer during what is always the most challenging time for a professional sportsperson.”
A Pompey XI were beaten 2-0 in a pre-season friendly at Chelsea U23s after conceding a couple of late goals.
Cowley said: “It was a useful exercise for us and we were able to get minutes into the legs of guys who needed that.
“There would have been some academy players involved, but they’re having to self-isolate so we saw a lot of trialists.
“Once the senior professionals went off, it was a very young team in the second half and we had to adapt.
“I felt there were some encouraging performances for us and we just need to be a bit more clinical to take some of our chances.”