The 28-year-old frontman will be out of contract at the end of the season following a disappointing 2020/21 campaign.
Assombalonga hasn’t started a game since February but surprisingly came off the bench for Yannick Bolasie in the final 10 minutes against QPR last weekend.
When asked about Assombalonga ahead of his side’s trip to Rotherham, Warnock replied: “He was brilliant last week, he was probably the highlight.
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“I thought he was outstanding last week. I said to him after training, I wondered if his next club was watching him in training.
“Unfortunately it didn't work out when he came on.”
Assombalonga is one of five Boro players who will be out of contract this summer, along with Ashley Fletcher, Marvin Johnson, Nathaniel Mendez-Laing and goalkeeper Jordan Archer
Three more Boro players – Bolasie, Neeskens Kebano and Marcus Bettinelli – are nearing the end of their loan deals, with plenty of changes expected at the Riverside.
When asked if he knows which players he wants to keep and allow to leave, Warnock said: “Absolutely, there are a couple depending on who we get in and if they wanted to stay.
“I’ve got an open mind now, we have got to start moving which we are doing. We had another few hours yesterday afternoon and I came back last night. We are looking all the time to try and get things.
“Most teams you can’t sign players until July the 1st if their contracts are up but I need players who will give me their word that they’re signing, that’s what I like to do. If they give me their word they are going to sign.
“If they did go anywhere else I didn’t want them anyhow because they have let me down and if they’ve let me down there they’ve let me down on the pitch. I don’t worry about things like that.
“If a player shakes my hand or gives me a word he’s coming then I can put that to bed, I think they’re coming.
“That’s what I want to do in the next couple of weeks, get three or four players agreed verbally with me that they are going to.”