Citi, the US bank, says taking over TalkTalk would have a limited impact on BT's financials, and it is surprised that BT's shares have risen on the prospect.
The broker, which rates BT at 'sell' with a target price of 165p, wrote before the deal was confirmed.
It calculates that buying the broadband provider could lift BT's revenue and EBITDA (earnings before interest, tax, depreciation and amortisation) by 1% to 2%.
The deal would initially dilute normalised free cash flow, the cash left after investment, though Citi sees scope for it to turn mildly accretive over the longer term as synergies come through.
BT has now agreed a rescue deal to buy TalkTalk's consumer and wholesale businesses out of administration, at a total cost of around £400 million including transaction costs.
The Government stepped in to help pave the way for a sale to BT after TalkTalk's attempts to find a buyer for its two businesses had failed.
BT said the rescue will protect services for 2.5 million customers, including 1.5 million retail customers and one million wholesale customers.
About 900 jobs are protected.
TalkTalk generated around £1.2 billion in revenue over the past 12 months but was loss-making, according to BT.
It is thought BT agreed to pay around £100 million to Ares Management, TalkTalk's majority shareholder and creditor.
A public interest intervention notice will let the Government consider the wider public interest once the Competition and Markets Authority (CMA) has reported back on any competition concerns.
Allison Kirkby, BT's chief executive, called it a genuinely unprecedented situation in which millions of citizens and businesses were at risk had TalkTalk collapsed.
Citi is wary of the regulatory backdrop, pointing to the CMA's provisional findings on the proposed deal between Nexfibre and Netomnia, two fibre network operators.
It views those findings as negative for BT because of what they could mean for the long-term structure of the UK fixed-line market.
