Lampard scored his 202nd and 203rd goals for the club to break Bobby Tambling's record and lead Chelsea to a 2-1 victory over Aston Villa on Saturday.
The 34-year-old, whose current deal expires this summer, struck twice in the second half to overhaul Christian Benteke's early strike for Villa.
Following Lampard's man of the match display, Redknapp said it was "mind blowing" that Chelsea could allow the midfielder to leave on a free. But he believes Lampard is certain to stay at Stamford Bridge if, as many predict, Mourinho returns as manager this summer.
Redknapp told Sky Sports: "He [Lampard] is the best we have seen in the Premier League years at arriving in the box late. In Premier League goals in the last ten years he has got into double figures.
"That is absolutely incredible. He is a great professional, player and to even think that Chelsea are even contemplating letting him go as a free agent in a couple of weeks is mind blowing.
"I think if Jose [Mourinho] comes back Frank is an absolute certainty [to stay. He has just put his team into the Champions League there. How much is that worth to the club? Surely you would want to give him a year or two because of that.
"But he is not a charity case and he should not be, he should be given a lot more respect than he has been by Chelsea. People may say that they have paid him vast amounts of money but the goals he has contributed, the way he is around the training ground with young players looking up to people like him.
"They are a dying breed players like Frank Lampard. They are old school and teach people the right ethics. That is where they are going to miss him if they let him go because Chelsea's loss will be somebody else's gain."
Villa were in a strong position to claim the victory following Benteke's strike and Ramires' sending off before the break. But Benteke turned from hero to villain as he was dismissed following a second yellow card for a high challenge on John Terry.
Redknapp was in no doubt that the striker's indiscipline had cost Villa, and that it could cost them further as they play Wigan in their final match and still not certain of Premier League survival.
"At one stage this game was in their [Aston Villa's] hands," said Redknapp. "If they had kept Benteke on the pitch I am sure they would have won this match.
"It was a massive turning point in the game. Aston Villa lost their focal point and the player that took the pressure off them every time the ball went forward. They did miss him."
He added: "The problem now is Villa have to go to Wigan without him. He is this team to a certain extent. The young players look up to him so much. Every time that ball goes up to him he relieves so much pressure. He is a sensational footballer."