Southgate told BBC Radio 5 Live: "There are times when you know your best players are your best players. Other people might doubt it but you're going with them because you know that you can get that level of performance from them.
"At the moment, I couldn't be confident on the performances Kalvin's had in the last few weeks that he could go onto the field and do the job we know that he's capable of."
"So, there's absolutely no reason why he can't find that form. Nothing would give us more pleasure than having a Kalvin that's back in the rhythm of playing and back to full confidence.
"We will be a better team, squad, whatever that might be, if that's the case, so we're hoping he can find that.
"Maybe a moment like this where you think, well, that's perhaps the lowest it can get for him in terms of the last few months, now he can just go for it and be himself.
"I think he knows exactly what we think of him, I think we've shown that support.
"A good version of him is an important player for us and that's why we've supported him as we have. We don't have many players of that profile.
"Unfortunately, his form just hasn't been good enough. I've spoken to him about that, he understands."